A pyramid off three balls

Three spheres with the radius R are lying on a plane, so that they all tangent each other. A forth Sphere, also with the radius of R, are lying upon the three others, (so that a pyramid arises). Compute the length between the middle point of the fourth sphere to the underlying plane, (whereas we assume the friction to be big enough to prevent the speheres from gliding).

Anyone?

This is for our repetetition course of the high school-math given at the university, so I thought this was the right place even though I was somewhat unseure.