Hi I have problems with exercise

Let $\displaystyle p(x) = x^3+x^2-2x-1$ and let $\displaystyle k = \mathbb{Q}$ [$\displaystyle x$] / $\displaystyle (p)$ Show that, in $\displaystyle K$,

$\displaystyle p(x^2-2) = p(x^3-3x) = 0$

Hence the three roots of $\displaystyle p $in $\displaystyle K$ are $\displaystyle x + (p)$, $\displaystyle x^2-2+(p)$ and $\displaystyle x^3-3x + (p)$

I do not know how to start

Thanks