Answer by taking a quote from the following article:

A Night at the Opera is the fourth studio album by the British rock band Queen, released on 21 November 1975 by EMI Records in the United Kingdom and by Elektra Records in the United States. Produced by Roy Thomas Baker and Queen, it was the most expensive album ever recorded at the time of its release. The album takes its name from the Marx Brothers film of the same name, which the band watched one night at the studio complex when recording. A Night at the Opera incorporates a wide range of styles, including ballads, songs in a music hall style, hard rock tracks and progressive rock influences.

"Death on Two Legs" can be referred to as Freddie Mercury's hate letter to Queen's first manager, Norman Sheffield, who for some years was reputed to have mistreated the band and abused his role as their manager from 1972 to 1975. Sheffield denied the allegations in his 2013 autobiography entitled "Life on Two Legs: Set The Record Straight", and referred to copies of the original 1972 management contracts between Sheffield and Queen, which were included in the book as proof of his defence. Though the song never makes direct reference to him, after listening to a playback of the song at Trident Studios during the time of album release, Sheffield was appalled, and sued the band and the record label for defamation, which resulted in an out-of-court settlement, but also confirmed his connection to the song.  During live performances, Mercury would usually rededicate the song to "a real motherfucker of a gentleman", although this line was censored on the version that appeared on their Live Killers album in 1979. Other than on the live album, he said it was dedicated to a "motherfucker I used to know".  In the Classic Albums documentary about the making of A Night at the Opera, Brian May stated that the band was somewhat taken aback at first by the bitterness of Mercury's lyrics, and described by Mercury as being "so vindictive that he [May] felt bad singing it". After the song came together, it was agreed that the "author should have his way", and the song was recorded as written.  As with "Bohemian Rhapsody", most of the guitar parts on this song were initially played on piano by Mercury, to demonstrate to May how they needed to be played on guitar. "Death on Two Legs" remained on the setlist until, and well into, The Game Tour in 1980, and was then dropped. However, the piano introduction was played during the Hot Space and Works tours.  "I'm in Love with My Car" is amongst Roger Taylor's most famous songs in the Queen catalogue. The song was initially taken as a joke by May, who thought that Taylor was not serious when he heard a demo recording.  Taylor played the guitars in the original demo, but they were later re-recorded by May on his Red Special. The lead vocals were performed by Taylor on the studio version, and all released live versions. The revving sounds at the conclusion of the song were recorded by Taylor's then current car, an Alfa Romeo. The lyrics were inspired by one of the band's roadies, Johnathan Harris, whose Triumph TR4 was evidently the "love of his life". The song is dedicated to him, the album says: "Dedicated to Johnathan Harris, boy racer to the end".  When it came down to releasing the album's first single, Taylor was so fond of his song that he urged Mercury (author of the first single, "Bohemian Rhapsody") to allow it to be the B-side and reportedly locked himself in a cupboard until Mercury agreed. This decision would later become the cause of much internal friction in the band, in that while it was only the B-side, it generated an equal amount of publishing royalties for Taylor as the main single did for Mercury.  The song was often played live during the 1977-81 period. Taylor sang it from the drums while Mercury played piano and provided backing vocals. It was played in the Queen + Paul Rodgers Tour in 2005 and the Rock the Cosmos Tour in 2008. Taylor would again play the song for his concerts with The Cross and solo tours, where instead of drums he played rhythm guitar.  "'39" was May's attempt to do "sci-fi skiffle". "'39" relates the tale of a group of space explorers who embark on what is, from their perspective, a year-long voyage. Upon their return, however, they realise that a hundred years have passed, because of the time dilation effect in Einstein's special theory of relativity, and the loved ones they left behind are now all dead or aged.  May sings the song on the album, with backing vocals by Mercury and Taylor. During live performances, Mercury sang the lead vocal. May had asked bassist John Deacon to play double bass as a joke but a couple of days later he found Deacon in the studio with the instrument, and he had already learned to play it.  Since Queen had named their albums A Night at the Opera and A Day at the Races after two of the Marx Brothers' most popular films, surviving brother Groucho Marx invited Queen to visit him at his Los Angeles home in March 1977 (five months before he died). The band thanked him, and performed "'39" a cappella.  George Michael performed "'39" at the Freddie Mercury Tribute Concert on 20 April 1992. Michael cited this song as his favourite Queen song, claiming he used to busk it on the London Underground.  Recently, Queen have included the song on the setlists of their recent tours with Adam Lambert  and before Adam with Paul Rodgers; for all these tours since 2005 it is sung as it is on the album by May.

Who composed it?
May sings the song on the album, with backing vocals by Mercury and Taylor. During live performances, Mercury sang the lead vocal.