Let G and H be two groups. If f: G H is a homomorphism, a G and b = f(a). If ord(a) = n, ord(b) = m, then n is a multiple of m. (Let be the identity of G and be the identity of H)

I know = ?

and that = ?

I have to prove from that given that n is a multiple of m.

Can I have some help with this proof? Thanks.