so hmmm check the picture below
the area of that triangular pyramid, will be the area of each of those triangles
notice, you have 4 triangles
2 triangles have a base of 18 and a height of 8
one triangle has a a height of 8 and a base of half 12
and the triangle, at the bottom has a base of 12 and an altitude or height of 18
now. to find the missing side in each right-triangle, simply use the pythagorean theorem
recall, the area of a triangle is 1/2 bh
so.. all you need is the base and height of each triangle, get the area of each, sum them up, that'd be the area of the triangular pyramid