Question:

Suppose f and g are isomorphisms from U to V. Prove or disprove each of the following statements:

a) The mapping f + g is an isomorphism from U to V

I have no idea where to start. Do I need to show that f(u) + g(u) = (f+g)(u)? Do I need to show that (f+g) is 1-1 and onto?