Is this correct? (Horizontal tangent)

In which point do the tangent have an horizontal tangentplane?
z = x^2 + 9^2/9 - 9

My solution is (0, 0, -9) - is this right?
Here is my math:
http://i.imgur.com/lr7vSru.png

I dont *quite* understand it, but the way I understand it, the plane where this equation (elliptic paraboloid) has an horizintal tangentplane, is on the top/bottom of it - which , I believe would be when Z_x = 0 and Z_y = 0.
No? Yes?

Question

Is this correct? (Horizontal tangent)

In which point do the tangent have an horizontal tangentplane? 
z = x^2 + 9^2/9 - 9

My solution is (0, 0, -9) - is this right?
Here is my math: 
http://i.imgur.com/lr7vSru.png

I dont *quite* understand it, but the way I understand it, the plane where this equation (elliptic paraboloid) has an horizintal tangentplane, is on the top/bottom of it - which , I believe would be when Z_x = 0 and Z_y = 0.
No? Yes?

anonymous · Answer

Bump x6