whats the easiest way to do this? i have never done this type of problem before
the easiest way?
consider the roots of the polynomial: $\displaystyle x^3 - 5 = 0$
Now apply the rational roots theorem. What can you come up with?
Alternatively, prove this lemma: If $\displaystyle 5 | x^3$, then $\displaystyle 5 | x$.
then apply the technique used here in post #2
it is a longer, more sophisticated proof, but it is the conventional one, and certainly one you should know, for historical and aesthetic reasons at least