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