2x²+xy-y²-4x+5y-6
=(2x-y)(x+y)-4x+5y-6
=(2x-y)(x+y)+2x+2y-6x+3y-6
=(2x-y)(x+y)+2(x+y)-3(2x-y)-6
=［(2x-y)(x+y)+2(x+y)］-［3(2x-y)+6］
=(2x-y+2)(x+y)-3(2x-y+2)
=(2x-y+2)(x+y-3)
5+7x-6x² =（5-3x）（1+2x）