Surface of a Pyramid

A right square pyramid's net is a square with a triangle on each side, like a star. Each triangle has the base edge for its base, and its height is the slant height, the line down the middle of the face.

This pyramid has base edge 6 and slant height 5. The base is ${6}^{2} = 36$. Each triangle is $\frac{1}{2}\cdot 6\cdot 5 = 15$, and four of them make 60. The surface area is $36+60 = 96$.

The rule is Surface area of a pyramid: $SA = {b}^{2}+2\cdot b\cdot s$, the base plus four half rectangles. Here the slant height is the one to use, not the height: the faces lean, and their own height is the slant.