x^2 - 3x - 3y - y^2 =
x^2 - y^2 - 3x - 3y =
(x^2 - y^2) - 3(x + y) =
(x - y)(x + y) - 3(x + y) =
((x - y) - 3)(x + y) =
(x - y - 3)(x + y);
x^2-3x-3y-y^2=-(y-x+3)*(y+x)=(x-y-3)*(y+x)
