Some suggestions for how to find a counterexample for this:
You need to find a number whose cube is not greater than the number. To not be greater, the cube should either be equal to the number or less than the number.
One of the common difficulties for finding a counterexample for many of these types of problems is the tendency to look at simple natural numbers (5, 7, 10, etc). Try a couple unusual numbers (like negative numbers, fractions smaller than 1, or even 0 or 1) see if any of these results in a cube (the result of multiplying by itself three times) that is equal to the original number or less than the original number. Any number that does that is a valid counterexample!
