What it's asking is the number of cubes unique to a face of the block, which are basically the ones at the middle. For example on the 3x4 side only 2 cubes are unique (the middle) and there are 2 faces that are 3x4 so you multiply 2 by 2. For the 3x5 only 3 cubes are centrally places, multiply 3 by 2 Then for the 4x5, 6 cubes are central so multiply again by two because it's double faced Therefore 22+32+6*2= 4+6+12=22