Showing that four stationary points of a multivariable function are at specific points.

A surface is described by the multivariable function f(x,y) where:

f(x,y) = x^3 + y^3 + 9(x^2 + y^2) + 12xy

a) Show that the four stationary points of this function are located at:

(x1, y1) = (0, 0)
(x2, y2) = (-10, -10)
(x3, y3) = (-4, 2)
(x4, y4) = (2, -4)