16x(x-y)^4 - 81x(x+y)^4
=x[16(x-y)^4 - 81(x+y)^4]
=x[4(x-y)^2+9(x+y)^2][4(x-y)^2 -9(x+y)^2]
=x[4(x-y)^2+9(x+y)^2][2(x-y)+3(x+y)][2(x-y) -3(x+y)]
=x(4x^2-8xy+4y^2+9x^2+18xy+9y^2)(2x-2y+3x+3y)(2x-2y -3x-3y)
=x(13x^2+10xy+13y^2)(5x+y)(-x-5y)
=-x(13x^2+10xy+13y^2)(5x+y)(x+5y)