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