You already know that 弦楽 means "string music," so add some pipes to that, like trumpets and saxophones, and you'll have orchestral music. You can't have a whole orchestra with just the string section!

This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own. 楽 has two on'yomi readings, but you know it's がく here because it's about 音楽 (music)!