Complex sentence: On April 9,2015, Primary released his mini-album, "2-1" on which Rap Monster, along with Kwon Jin Ah, was with Primary for the song, "U".
Split into simpler sentences: On April 9,2015, Primary released his mini-album, "2-1". It was revealed that Rap Monster, along with Kwon Jin Ah, collaborated with Primary for the song, "U".