You may have looked at the face of the prism and thought to subtract the dimensions of the smaller square from the dimensions of the larger square, and then multiply those differences together. However, this will not give you the correct volume. You must separately use $\displaystyle{V}={l}\cdot{w}\cdot{h}\usepackage{cmbright}\Huge\$ for the large prism and for the hole. Then subtract the smaller volume from the larger one.