I have to find the points on the sphere $\displaystyle x^2+y^2+z^2=36$ that are closest to and furthest from the point $\displaystyle (1,2,2)$

This is where I am up to. And I will show where I am stuck.

$\displaystyle \nabla f(x,y,z) = 2x+2y+2z$

What do I do now? Do I use the distance formula somehow?

Any help is appreciated!

thanks in advance!