well! actually i need some help because i can really solved this?
It isn't your fault. The problem statement is flawed as I demonstrated. Unfortunately I haven't a clue what was intended. Where did you get the problem from?
I believe what was intended is that "if $\displaystyle n^2$ is divisible by prime number a then n is divisible by a". The simplest way to prove that is to look at the prime fatorizations of n and $\displaystyle n^2$.