Covariance for a bivariate normal distribution
Once you proved that $\mathbb{Cov}[Y-\rho X,X]=0$ you are done because (using the fact given in the text) this is equivalent to prove that $(Y-\rho X)\perp\!\!\!\perp X $ thus
$$\mathbb{E}[(Y-\rho X)^{10}X^3]=\mathbb{E}[(Y-\rho X)^{10}]\cdot\mathbb{E}[X^3]=0$$
Being $\mathbb{E}[X^{2n+1}]=0$, $ \forall n$