Polynomial multiplication coefficients
Yes, there are more efficient ways.  If you have two polynomials $p(x)=\sum p_i x^i$ and $q(x) = \sum q_i x^i$, then the coefficient of $x^k$ in $p(x) q(x)$ is given by

$$\sum_{i=0}^k p_i q_{k-i}.$$

This can be computed in $O(k)$ time, which is more efficient than multiplying the polynomials and extracting the coefficient of $x^k$.

With preprocessing, you can just multiply the polynomials during the preprocessing phase and store all of the coefficients.  Then looking up any desired coefficient can be done in $O(1)$ time.