(x-2)+x^2(2-x)
=(x-2)-x^2(x-2)
=(x-2)(1-x^2)
=(x-2)(x+1)(x-1）
3a(x-y)+9(y-x)
=3a(x-y)-9(x-y)
=3(x-y)(a-3)
(2m-3n)^2-(2m-3n)
=(2m-3n)(2m-3n-1)