Yeah I think I got it now, I set the gradient equal to the normal of that other plane.
\nabla f \left(x_0,y_0,z_0\right)= u=<1,2,3>
then I set the partials equal to that normal and I get f_x=2x_0; f_y=-1; f_z=2z_0
so... \nabla f \left<2x_0,-1,2z_0\right>=<1,2,3>
then my only problem is my...