Description: In this example of evaluating expressions, we're dusting off some geometry. On top of that, it's a word problem. We're seeing how different concepts in math are layered on top of each to create more interesting and complex problems to solve. Created by Sal Khan. The surface area of a cube is equal to the sum of the areas of its six sides. The surface area of a cube with side length x is given by the expression 6x squared. The area of each side is going to be x times x is x squared, and there's six of them.*Teacher Advisor is 100% free.