\[
\boxed{
{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right) =-{\frac { \left( 3\,{x}^{2}+a \right) {\frac {\rm d}{{\rm d}x}}y \left( x \right) }{{x}^{3}}}-{\frac {by \left( x \right) }{{x}^{6}}}=0}
\]