3a(x-y)+9(y-x)
=3a(x-y)-9(x-y) 提取公因式3(x-y)
=3(x-y)(a-3)
(2m-3n)²-2m+3n
=(2m-3n)²-(2m-3n) 提取公因式(2m-3n)
=(2m-3n)(2m-3n-1)