x^2 + xy - 2y^2 =
x^2 + 2xy - xy - 2y^2 =
(x^2 - xy) + (2xy - 2y^2) =
x(x - y) + 2y(x - y) =
(x+2y)(x-y)
x^2 + xy - 2y^2 =
x^2 + 2xy - xy - 2y^2 =
(x^2 - xy) + (2xy - 2y^2) =
x(x - y) + 2y(x - y) =
(x+2y)(x-y)