Question 6748
The formula is:


SA=2(lw)+2(lh)+2(wh)


You're basically finding the area of all six sides of the rectangular prism.  
In order to find the area of a rectangle, you multiply length by width.  
But a prism has dimension to it so you have to find the area of all the sides. 


to find the area of the top and bottom, multiply the length and width.  when you get that answer, multiply that by 2 because you have 2 faces that are exactly the same. 


to find the area of the front and back faces, multiply the length and the height, then multiply that by 2 because again, you have two faces that are identical.


to find the area of the side faces, multiply the width and the height, and multiply that by 2, because there are two side faces. 


after multiplying them, add up their value.  That will give you the surface area of the rectangular prism.