\[ a^2 \sin (y(x))-b f(x)+y''(x)=0 \] ✗ Mathematica : cpu = 0.0367806 (sec), leaf count = 0 , could not solve
DSolve[-(b*f[x]) + a^2*Sin[y[x]] + Derivative[2][y][x] == 0, y[x], x]
✗ Maple : cpu = 0. (sec), leaf count = 0 , could not solve
dsolve(diff(diff(y(x),x),x)+a^2*sin(y(x))-b*f(x)=0,y(x))