Exact Equations & Substitutions · Hard problem

The equation $(6 x y - 3)\,dx + (3 x^{2} + 6 y)\,dy = 0$ is exact. Its solution through $(0, -1)$ is $\psi(x, y) = C$ for a potential $\psi$ with $\psi(0, 0) = 0$. Find $C$.