Partial Derivatives & the Gradient · Medium problem

Let $f(x,y) = x^{3} - xy^{2}$. Use the linear approximation of $f$ at $(3,2)$ to estimate $f(3.1,\ 1.8)$, then find how much that estimate differs from the true value of $f(3.1,1.8)$ (estimate minus true value). Give an exact answer as a decimal.