\[ -a x-b \sin (x)-c \cos (x)+y^{(n)}(x)+2 y^{(3)}(x)+y'(x)=0 \] ✓ Mathematica : cpu = 0.82157 (sec), leaf count = 80
DSolve[-(a*x) - c*Cos[x] - b*Sin[x] + Derivative[1][y][x] + 2*Derivative[3][y][x] + Derivative[5][y][x] == 0,y[x],x]
\[\left \{\left \{y(x)\to \frac {1}{16} \left (8 a x^2+\cos (x) \left (b \left (2 x^2-9\right )-2 (5 c x+8 (c_4 x-c_2+c_3))\right )+\sin (x) \left (-6 b x+c \left (13-2 x^2\right )+16 (c_2 x+c_1+c_4)\right )\right )+c_5\right \}\right \}\] ✓ Maple : cpu = 0.419 (sec), leaf count = 69
dsolve(diff(diff(diff(diff(diff(y(x),x),x),x),x),x)+2*diff(diff(diff(y(x),x),x),x)+diff(y(x),x)-a*x-b*sin(x)-c*cos(x)=0,y(x))
\[y \left (x \right ) = \frac {\left (b \,x^{2}+\left (-4 c -8 c_{4}\right ) x -6 b -8 c_{2}+8 c_{3}\right ) \cos \left (x \right )}{8}+\frac {\left (-c \,x^{2}+\left (-4 b +8 c_{3}\right ) x +6 c +8 c_{1}+8 c_{4}\right ) \sin \left (x \right )}{8}+\frac {a \,x^{2}}{2}+c_{5}\]