The answer is:
X=20
This is because 20-7=13 and makes a true statement
Hope this helps have a good day! :)
Once again the answer is 0. The reason is because the cube is 3 by 3 there is a cube 1 by 1 in the center of the big cube, because it is surrounded by other 1 by 1 cubes that cube doesn't get any sides painted. Therefor there is no possibility for all cubes to have at least one side painted.
The answer you are looking for is 4x-5