(1/3a^2b)^2*(-9ab^3)/(-1/2a^4b^2)
=(1/9a^4b^2*-9ab^3)/(-1/2a^4b^2)
=2ab^3
4x^2(x-y)+(y-x)
=(x-y)(4x^2-1)
=(x-y)(2x-1)(2x+1)