Vector Spaces & Bases · Challenge problem

In $P_6$ (real polynomials of degree at most $6$) let $U$ be the subspace of even polynomials ($p(-x) = p(x)$) and let $W = \{p\in P_6 : p(1) = 0 \text{ and } p(3) = 0\}$. Find $\dim(U + W)$.