Stringed Instrument
noun
You already know that 楽器 means "musical instrument" so adding a couple of bowstrings to it will make it a stringed instrument.
げんがっき
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
私の叔父は、弦楽器の修理をしています。
My uncle repairs stringed instruments.