(x-y)(3x-7y)²-4(x-y)(x+y)²
=(x-y)[(3x-7y)²-4(x+y)²]
=(x-y)(3x-7y-2x-2y)(3x-7y+2x+2y)
=(x-y)(x-9y)(5x-5y)
=5(x-y)²(x-9y)