$3(x+y)(x-y)$ + $(x-y)^{2}$ - $(x+y)^{2}$
$3($$x^{2}$-$y^{2})$ $+$ $(x-y)^{2}$ - $(x+y)^{2}$
$3x^{2}$ - $3y^{2}$ $+ $$x^{2}$-$2xy$+$y^{2}$ - ($x^{2}$+$2xy$+$y^{2}$)
$3x^{2}$ - $3y^{2}$ $+ $$x^{2}$-$2xy$+$y^{2}$ - $x^{2}$-$2xy$-$y^{2}$
$3x^{2}$ - $3y^{2}$ - $4xy$
Xin ctrlhn cho nhóm ạ