This is how I chose to do this:
Split the shape into three: a rectangle and two triangles. Then find the area of each.
Rectangle: 12 x 6 = 72
Triangles (if you take both triangles, you can make a square, that's what I'm doing): 18 - 12 = 6; 6 x 6 = 36
Add the two to get the complete area: 72 + 36 = 108
Your answer will be 108 feet³.
Hope this helps,
♥A.W.E.S.W.A.N.♥