? [Hint:
      Draw the surface, and at the point $(2,1,4)$, draw these
      vectors. See the Sage plot. Think about how the vectors are
      telling you about how changes in the inputs are related to
      changes in the outputs.]</li>

      <li>The vectors above are tangent to the surface. Use them to
      obtain a normal vector to the tangent plane, and then give an
      equation of the tangent plane. (You should compare it to your
      equation from <a href=
      "#tangent-plane-9-x2-y2">this pro