\[ -a x-b \sin (x)-c \cos (x)+y^{(n)}(x)+y'(x)+2 y^{(3)}(x)=0 \] ✓ Mathematica : cpu = 0.818077 (sec), leaf count = 80
\[\left \{\left \{y(x)\to \frac {1}{16} \left (8 a x^2+\cos (x) \left (b \left (2 x^2-9\right )-2 (5 c x+8 (c_4 x-c_2+c_3))\right )+\sin (x) \left (-6 b x+c \left (13-2 x^2\right )+16 (c_2 x+c_1+c_4)\right )\right )+c_5\right \}\right \}\] ✓ Maple : cpu = 0.978 (sec), leaf count = 69
\[ \left \{ y \left ( x \right ) ={\frac { \left ( b{x}^{2}+ \left ( -4\,c-8\,{\it \_C4} \right ) x-6\,b-8\,{\it \_C2}+8\,{\it \_C3} \right ) \cos \left ( x \right ) }{8}}+{\frac { \left ( -c{x}^{2}+ \left ( -4\,b+8\,{\it \_C3} \right ) x+6\,c+8\,{\it \_C1}+8\,{\it \_C4} \right ) \sin \left ( x \right ) }{8}}+{\frac {a{x}^{2}}{2}}+{\it \_C5} \right \} \]