y=x^2/(2x+1)^3
y' = { (2x+1)^3 * (x^2)' - x^2 * [(2x+1)^3]' } / [(2x+1)^3]^2
= { (2x+1)^3 * 2x - x^2 * 3(2x+1)^2 * (2x+1)' } / [(2x+1)^3]^2
= { 2x * (2x+1)^3 - x^2 * 3(2x+1)^2 * 2 } / (2x+1)^6
= { 2x * (2x+1)^3 - 6x^2 * (2x+1)^2} / (2x+1)^6
= {2x * (2x+1)^2 * [(2x+1) - 3x ]} / (2x+1)^6
= {2x* [2x+1 - 3x ]} / (2x+1)^4
= -2x* (x-1) / (2x+1)^4