Answer
$(x-3y)(x+3y)(x-y)(x+y)$
Work Step by Step
Treat as a trinomial of the form $t^{2}+bt+c,$ where $t=x^{2}$
$=t^{2}-10y^{2}\cdot t+9y^{4}$
we search for factors of $c=9y^{2}$ whose sum is $b=-10y^{2}$
and we find $-9y^{2}$ and $-y^{2}$
$= (t-9y^{2})(t-y^{2})\quad$ ... bring back x,
$=(x^{2}-9y^{2})(x^{2}-y^{2})\quad$
recognize two differences of squares
= $(x-3y)(x+3y)(x-y)(x+y)$
