a(x-y)+b(x-y)^2+xy(x-y)^3

a(x-y)+b(x-y)²+xy(x-y)³=ax-ay+b(x²-2xy+y²)+xy(x³-3x²y+3xy²-y³)= x^4y-3x²y²+3x²y³-xy^4+bx²-2bxy+by²+ax-ay