so. hmmmm notice the first picture, that's a rectangular prism
is really just 6 rectangles stacked up to each other at the edges
now, notice the second picture, is a prism/shelf with a length of 48, width of 12 and height of 1
so, your shelf is really just 6 rectangles stacked up to each other at the edges
you have 2 rectangles of 48x1 on the sides
2 rectangles of 12x1 for the front and back
and 2 rectangles of 12x48, for the top and bottom
get the area of each of those rectangles, add them up, and that's the surface area of the shelf
