How do you factor x^6/8-y^3/27?

Let ${x}^{2} = X ,$ then
${\left(\frac{X}{2}\right)}^{3} - {\left(\frac{y}{3}\right)}^{3}$ = $\left(X - y\right) \left({X}^{2} + X y + {y}^{2}\right)$=
= $\left({x}^{2} - y\right) \left({x}^{4} + {x}^{2} y + {y}^{2}\right)$