Tìm f: $\mathbb{R}\rightarrow \mathbb{R}$: $f\left ( x^{3} \right )-f\left ( y^{3} \right )=\left ( x^{2}+xy+y^{2} \right )\left ( f\left ( x \right )-f\left ( y \right ) \right )\forall x, y\in \mathbb{R}$