Any maths buffs?

I have a question that is puzzling me...

Show that the expression

                f(x,y)= x^2−2xy+2(y^2)−6y+3

has a stationary point at (x,y) = (3,3). What type of stationary point is it?

so for the derivative i get

dy/dx=2x/(2x-4y+6) which would give an infinite gradient (dy/dx=6/0) therefore not being a stationary point?

thanks for any help.

sorry if i missed something.