Hi.
Please help, I know absolutely nothing. So need someone with a little patience.
Just bought a bike of ebay, the seller described as

The rear plastic part (front derailleur) on the gear is broken so no longer able to ride.

The chain is loose. I know which bits the front d and rear d is but how do I know which is broken? Can't find how to
upload a picture for some advice. Might not be a lot to go on but hope someone can help.
Thanks in advance