Exact Equations & Substitutions · Easy problem

For the equation $(3x^2y + y^3)\,dx + (x^3 + 2xy^2)\,dy = 0$, write $M = 3x^2y + y^3$ and $N = x^3 + 2xy^2$. Evaluate $M_y - N_x$ at the point $(1, 2)$.