1. The problem statement, all variables and given/known data That's what I'm supposed to prove. 2. Relevant equations A function f is an injection if f(x1)=f(x2) ---> x1=x2 3. The attempt at a solution I'm just having trouble constructing a formal proof. I mean, it's obvious that f(g(x1))=f(g(x2)) then either g(x1)=g(x2) or g(x1)≠g(x2), and since g(x1)≠g(x2) implies x1≠x2, ...... I don't know. I'm getting all tongue-tied. Help me out here.