(x-1)(3x-7y)^2+4(1-x)(x+y)^2
=(x-1)(3x-7y)^2-4(x-1)(x+y)^2
=(x-1)[(3x-7y)^2-4(x+y)^2]
=(x-1)[3x-7y+2(x+y)][(3x-7y)-2(x+y)]
=(x-1)(5x-5y)(x-9y)
=5(x-1)(x-y)(x-9y)