y'=\(\dfrac{\left(-cosx\right)'.3.sin^3x-\left(-cosx\right)\left(3.sin^3x\right)'}{\left(3.sin^3x\right)^2}\)
y'=\(\dfrac{3.sinx.sin^3x+9.cosx.sin^2x}{^{ }\left(3.sin^3x\right)^2}\)
y'=\(\dfrac{3.sin^4x+9.cosx.sin^2x}{9.sin^6x}\)
y'=\(\dfrac{3.sin^2x\left(sin^2x+3.cosx\right)}{9.sin^6x}\)
y'=\(\dfrac{sin^2x+3.cosx}{3.sin^4x}\)