\[ \boxed { {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) +{a}^{2}\sin \left ( y \left ( x \right ) \right ) -b\sin \left ( x \right ) =0} \]
Mathematica: cpu = 0.050006 (sec), leaf count = 23 \[ \text {DSolve}\left [a^2 \sin (y(x))-b \sin (x)+y''(x)=0,y(x),x\right ] \]
Maple: cpu = 1.248 (sec), leaf count = 0 \[ \text {could not solve} \]