That's a pretty straight forward problem, isn't it? The gradient of f(x, y, z) where [tex]f(x, y, z)= x(x^2+ y^2)^{1/2}- z[tex] is a vector perpendicular to the surface. And once you have vector <A, B, C> perpendicular to the surface at , the tangent plane is given by . Have you calculated that?