The volume of a triangular prism with a known base and height is the same as half the volume of a rectangular prism with the same base and height (since a right triangle is simply a rectangle sliced diagonally).
So, we will first solve this as if it were a rectangular prism, finding volume accordingly.
4 * 8 = 32
32 * 24 = 768
Now, we know what the volume of a rectangular prism of those dimensions would be, so all we have to do is divide by 2 to find the volume of the triangular prism that is the soccer goal.
768 / 2 = 384
The volume of the soccer goal is 384 cubic feet.
Hope that helped! =)