I have 2 I am struggling with:

1. Let f:A to B. Prove that f is a bijection iff for all b that is an element of B, there exists exactly one a that is an element of A f(a)=f(b).

2. Prove: If f:A to B is a bijection and g:B to C is a bijection, then the composition of g of f is a bijection.