Consider that the pyramid has 4 triangular sides and one rectangular (not square) side. Call the height of the pyramid "H", the length of the rectangle "L" and the width "W".

Area = L*W + (L*H) + (W*H)

Normally a triangle's area is (W*H)/2, but since we have a rectangle for a base we have to compute the area of two pairs of equal triangles.

Area = L*W + (L*H) + (W*H)

Normally a triangle's area is (W*H)/2, but since we have a rectangle for a base we have to compute the area of two pairs of equal triangles.