原式=(2x-y)(x+y)-4x+4y-6 =(2x-y+2)(x+y-3)
2x^2+xy-y^2-4x+5y-6=（x+y）(2x-y)-4x+5y-6 =（x+y）(2x-y)+2(x+y)-6x+3y-6=(x+y)(2x-y+2)-3(2x-y+2)=(2x-y+2)(x+y-3)
（2x-y+2）(x+y-3)
