Solution for the equation

Taking x = tan A
           A = tan^-1x

y= tan^-1(3tanA - tan³A/1 - 3tan²A)                          
y=tan^-1(tan3A)                                      {tan-1(tanx) =x}
y=3A .
y = 3tan^-1x
Differentiating wrt x

dy/dx = 3 (1/1+x²)                                {d(tan^-1x)/dx = 1/1+x² }