[(x-y)^7] ÷ [(y-x)^6] + [(-x-y)^3] ÷ (x+y)
=[(x-y)^7] ÷ [(x-y)^6] + [-(x+y)^3] ÷ (x+y)
=(x-y) - (x+y)²
=x-y-x²-y²-2xy