Prove that polynomial is not a square of an integer

[VanBuren]

Integers $\displaystyle a, b, c$ are given. Prove that there is positive integer $\displaystyle x$ such, that $\displaystyle x^3 + ax^2 + bx + c$ is not a square of an integer.
Well... expression isn't a square of an integer if it can by divided by $\displaystyle k$, but not by $\displaystyle k^2$. But I don't know how to use it in this problem. I even tried to factorize the polynomial, without effect.