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