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