ab(x+y)(x-y)-xy(a+b)(a-b)
=ab(x^2-y^2)-xy(a^2-b^2)
=abx^2-aby^2-xya^2+xyb^2
=ax(bx-ay)-by(ay-bx)
=ax(bx-ay)+by(bx-ay)
=(ax+by)(bx-ay)