Plot3D[9x y-x^3-y^3-6,{x,-5,5},{y,-5,5},RegionFunction->Function[{x,y,z},.5z^2+x^2+y^2≤500],AxesEdge->{{-1,-1},{-1,-1},{-1, -1}},AxesLabel->Automatic, Boxed->False]