Prove that, for any positive integer $\displaystyle n$

$\displaystyle n(n-1)(n-2) - 6 \begin{bmatrix} \frac{n}{3} \end{bmatrix}$

is divisible by 9.

$\displaystyle \begin{bmatrix} x \end{bmatrix}$ denotes the integer part of $\displaystyle x$.

I wasn't sure how to go about this question, any help?