You don't want dy/dx you are finding a stationary point on a surface so you need to find the point at which the function has zero derivative w.r.t x and w.r.t y.
So take the partial derivative of f(x,y) w.r.t x and the partial derivative w.r.t y
@TheBrick(Tommy) started
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You don't want dy/dx you are finding a stationary point on a surface so you need to find the point at which the function has zero derivative w.r.t x and w.r.t y.
So take the partial derivative of f(x,y) w.r.t x and the partial derivative w.r.t y