\[ \boxed { {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) =-4\,{\frac {\sin \left ( 3\,x \right ) y \left ( x \right ) }{ \left ( \sin \left ( x \right ) \right ) ^{3}}}=0} \]
Mathematica: cpu = 0.137517 (sec), leaf count = 70 \[ \left \{\left \{y(x)\to c_1 \sqrt [4]{\cos ^2(x)-1} P_{-\frac {1}{2}+4 i}^{\frac {i \sqrt {47}}{2}}(\cos (x))+c_2 \sqrt [4]{\cos ^2(x)-1} Q_{-\frac {1}{2}+4 i}^{\frac {i \sqrt {47}}{2}}(\cos (x))\right \}\right \} \]
Maple: cpu = 0.109 (sec), leaf count = 41 \[ \left \{ y \left ( x \right ) ={\it \_C1}\,\sqrt {\sin \left ( x \right ) }{\it LegendreP} \left ( -{\frac {1}{2}}+4\,i,{\frac {i}{2}}\sqrt {47}, \cos \left ( x \right ) \right ) +{\it \_C2}\,\sqrt {\sin \left ( x \right ) }{\it LegendreQ} \left ( -{\frac {1}{2}}+4\,i,{\frac {i}{2}} \sqrt {47},\cos \left ( x \right ) \right ) \right \} \]