Let f: G --> H be an isomorphism, G and H are groups.

I've already showed that for every x in G, |f(x)| = |x|. (|x| = order of x, the smallest positive integer such that $\displaystyle x^n$ is the identity element)

How to show that any two isomorphic groups have the same number of elements of n, where n is any positive integer?

Please help. Thank you.