Hey there!
When we find the volume of a prism, we compute the equation:
V = lwh
This means that volume is equal to length times width times height.
Therefore, it looks as if we need three measurements, when we only have two, the length and height. However, by looking at this picture, we see that the width is actually the same as the length as it is a square, therefore our dimensions are:
V = 7 * 7 * 3
V = 49 * 3
V = 147 units cubed
Hope this helps!