How do you calculate the surface area of a rectangular prism?

You would add area of all the sides up.  Now, that's literally the formula I like to consider.  And, that will work for any non-circular prism, period.  
Now, special formulas do exist for specific types of prisms, for instance, a rectangular prism, you could consider the formula for the area of one side, length x width, then consider there is another one of those sides, so two of them, so 2 x length x width.  Then, pick another side and find that area, like width x height.  And, there is another one of those, so 2 x width x height.  And, likewise for length x height.  There would be 2 x length x height.  So, adding those up, you would have:total area rectangular prism = 2LW + 2LH + 2WH
where L = length, W = width, and H = height