The amount of wrapping paper you need is defined by the area of the prism's faces.
These are all rectangles so their area is equal to a*b where a is the length of the longest side and b that of the shortest.
We have two faces with sides 13in and 4in, two with sides 4in and 12in and two with sides 13in and 12in. Their areas are 13in*4in=52in^2, 12in*4in=48in^2 and 13in*12in=156in^2. Since we have two of each the total area is:
2*52in^2 + 2*48in^2 + 2*156in^2 = 512 in^2.
