x的平方减y的平方-3x+3y=(x^2-y^2)-3(x-y)=(x+y)(x-y)-3(x-y)=(x+y-3)(x-y)
x²-y²-3x+3y=(x²-y²)-3(x-y)=(x+y)(x-y)-3(x-y)=(x-y)(x+y-3)
3x-3y
