x²－2y²－xy－x－y
=(x²－y²)－(y²＋xy)－(x＋y)
=(x－y)(x＋y)－y(x＋y)－(x＋y)
=(x＋y)[(x－y)－y－1]
=(x＋y)(x－2y－1)