What is the surface area formula of a 3-dimensional rectangle?
For a rectangular prism with sides
This occurs since there are two pairs of three different faces on every rectangular prism.
Each pair of faces is a different rectangle using two of the the three dimensions of the prism as its own side.
One side is just
This could also be imagined as a series of flattened-out rectangles:
The blue rectangles are
The yellow rectangles are
The red rectangles are
Again, the surface area would be