check the picture below.
so the rectangular prism, or that box, is really just 6 rectangles stacked up to each other at the edges, notice
front and back are just two 9x5 rectangles
left and right are just two 5x4 rectangles
top and bottom are just two 9x4 rectangles
get the area of all 6, sum them up, and that's the area of the box.