\[
\boxed{
{\frac {\rm d}{{\rm d}x}}y \left( x \right) ={\frac {3\,{x}^{4}+3\,{x}^{3}+\sqrt {9\,{x}^{4}-4\, \left( y \left( x \right)  \right) ^{3}}}{ \left( 1+x \right)  \left( y \left( x \right)  \right) ^{2}}}=0}
\]