x^2+xy-2y^2-x+7y-6
=(x+2y)(x-y)-x+7y-6
x+2y -3
X
x-y 2
十字相乘得：2(x+2y)-3(x-y)=-x+7y是一次项
所以原式＝(x+2y-3)(x-y+2)