hey i have this problem where

z = xy(6-x-y) when multiplying out and differentiating with respect to x i get:

dz/dx = 6y - 2xy -y^2 and y dz/dy = 6x - x^2 - 2xy

I understand with the -2xy you can make the equations equal and find stationary points, which i got as (0,0) (6,0) and (0,6) but once checking the answers there was a further point (2,2) and I don't understand on how to obtain this stationary point, help would be much appreciated than you :)