The surface area of a cube is 6 times the surface area of one face.
If the side of the cube is x, then the surface area of one side is x^2, and the total surface area is 6x^2 (because there are 6 squares making up the surface of the cube).
Here, we have A(x) = 6 x^2, which DOES vary directly as the square of the length of an edge, denoted by x.