Hi!

I have a calculus exam tomorrow, and one of the questions in the 'Series Expansion for Functions of Two Variables' is confusing me a little...

"A Sphere is defined by the equation x^2 +y^2+z^2=9 Determine the equation of the tangent plane on its surface when x=1, and y=2"

the answer is given: x+2y+2z=9

I assumed I had to jiggle the equation, so i have z=-x-y+3, and start my partial differentiations from there, but my numbers end up silly, and the answer is way wrong when I do that.

Any help?