$\displaystyle x^4-12x^2y^2+16y^2$

the ans to this is

$\displaystyle (x^2 - 2 x y - 4 y^2) (x^2 + 2 x y - 4 y^2)$

but I don't what steps are taken to get this I tried U subst

but that did not seem to get this. looks like a plug in type solution

but textbook does not show one

thnx ahead