i have an exam coming up and this is going to be in it i have a problem in my notes i cant seem to do can anyone help please

Find the stationary points of the following surface. and determine thier nature of the stationary points.

f(x,y) = x^3 + y^3 - 3x - 12y +20

i understand the steps but for some reason when a Partial diff it never seems to come out like anything els in my notes any help would be very much apreciated