因式分解9(x-y)的平方+3x-3y
问题描述：
因式分解9(x-y)的平方+3x-3y
答
9(x-y)的平方+3x-3y
=9(x-y)的平方+3(x-y)
=3(x-y)[3(x-y)+1]
=3(x-y)(3x-3y+1)