partial derivative and finding an equation

Let P(x,y)= (xy)/(ax+by)

where P represents profit: x,y>0 represent sales of products X, and Y

a,b are constants>0

show that marginal profit when x=y equal to 1/(a+b)

what does it mean when x=y? finding partial deriv and set them = to each other doesnt work does it?

dp/dx =[ y(ax+by)- a(xy)]/ [(ax+by)^2]

dp/dy =[ x(ax+by) -b(xy)]/ [(ax+by)^2]

find the equation of the horizontal plane that is tangent to the graph

z= G(x,y)= x^2 -4xy - 2y^2 +12x - 12y -1

how do u do this question with 3 d graph?

i am (Headbang) right now please help