Well, you might start with the definition of "cubic residue". What is that?
Hi,
If you know a few facts, this is easy. Otherwise, it might be kinda hard.
The multiplicative group of is cyclic with generator, say x (a primitive root mod p). Then the order of is the number of elements in the set . Now if 3 divides p - 1, then p is congruent to 1 mod p, but p is congruent to 2 mod 3. So gcd(p-1,3)=1. That is, the order of is p - 1 and so every non-zero element of is a cube; i.e. a cubic residue.