Q: How am I going to write pow? It looks hard.
A: Define it in terms of multiplication of polynomials and define multiplication of polynomials using addition of polynomials. And addition of polynomials is pretty easy. You did something like it in Homework #3.
More: multiplication of polynomials also requires scaling. Suppose p is a polynomial and so is q. Suppose further than p = a0 + a1 x + ... an x^n. Then
p q = a0 q + x (a1 + a2 x + ... an x^(n-1)) q.So we see that if p is not empty (zero), then we can form the multiplication by a scaling of q by the first coefficient of p, and then adding it to the result of shifting (multiplying by x is easy) a recursive call with the rest of p with q.