I am having a difficult time understanding this. I have a question I need to solve.

Prove that if a divides bn and a,b are relatively prime then a divides n.

If I understand correct, for an integer to be relatively prime means that gcd{a,b}=1.

I think I also understand the idea. I just do not understand how to start a proof of this nature. I am really confused...

Any help would be appreciated. I do not want a complete answer, just some guidance.