Suppose that f:A->B and g:B->C. Prove that if f is not onto and g is one-to-one, then gof is not onto.

Am I right in saying that we need to find some x in B which shows that f is not onto? Then we can say that for any a in A, gof(a) does not equal g(x)? I don't know where to go from here or even if "here" is a starting point.

Thanks for any help!

Also, how do you use LaTeX in this forum? I've seen it used in other posts but I can't figure out how to do it.