4(3x-y)的平方-(x+3y)的平方
=(2(3x-y))²-(x+3y)²
=(6x-2y)²-(x+3y)²
=(6x-2y+x+3y)(6x-2y-x-3y)
=(7x+y)(5x-5y)
=5(7x+y)(x-y)