Exact Equations & Substitutions · Hard problem

The equation $(8 x y + 4)\,dx + (4 x^{2} + 2 y)\,dy = 0$ is exact. Its solution through $(3, 1)$ is $\psi(x, y) = C$ for a potential $\psi$ with $\psi(0, 0) = 0$. Find $C$.