Find the extremum of function z = XY (3-x-y), (x > 0, Y > 0)

Find the extremum of function z = XY (3-x-y), (x > 0, Y > 0)

To find the partial derivatives of X: &; Z / &; X = 3y-2xy-y ^ 2 to Y: &; Z / &; y = 3x-x ^ 2-2xy make two partial derivatives equal to 0 at the same time