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