A pyramid with height h and base an equilateral triangle with side a (a tetrahedron).

This is an example problem from the text. If someone could walk me through how to do this problem using integration, I think it will help me grasp how to do the other problems.

I'm having trouble starting, and not sure what I'm supposed to do. I think I'm supposed to find the surface area of the triangle with respect to height, and evaluate it from 0 to h. But how to I find out what the triangular base is at any given height?