The answer is D)6x2 square units.
A surface area of a cube is a sum of its sides' areas. A side of the cube is a square, and there are total 6 sides of the cube. Also, an area of the side of the cube is the area of the square, which can be expressed as a², where a is the length of the side. Therefore, the surface area (SA) of the cube is:
SA = 6*a² = 6a²
If side length is x, that means that a = x units.
After replacing it in the formula, you will have:
SA = 6a² = 6x² square units