Find the absolute maximum and minimum of f(x,y)=x^2+xy+y^2-6y on the rectangle {f(x,y) | -3 <= x <=3, 0 <= y <= 5}

i find the critical point by taking the derivative of f(x,y), setting it to 0, then finding x and y.

fx = 2x+y

fy= x+2y-6

y = -2x

x + 2(-2x) - 6 = 0

x - 4x = 6

-3x = 6

x = -2

y = -2(-2)

y = 4

i'm unsure of what to do next, can someone help? thanks!