81/8*(-x-y)^6*(x-y)^3*4/9(x+y)^4*(y-x)^2

81/8*(-x-y)^6*(x-y)^3*4/9(x+y)^4*(y-x)^2
=81/8*4/9*(-1)^6*(x+y)^6(x+y)^4(x-y)^3(x-y)^2
=9/2*(x+y)^5*(x+y)^5(x-y)^5
=9/2*(x+y)^5[(x+y)*(x-y)]^5
=9/2*(x+y)^5(x^2-y^2)^5
=9/2*(x^3-xy^2+x^2y-y^3)^5