\[ \boxed { 4\,{\frac {{\rm d}^{3}}{{\rm d}{x}^{3}}}y \left ( x \right ) -8\,{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) -11\,{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) -3\,y \left ( x \right ) +18\,{{\rm e}^{x}}=0} \]
Mathematica: cpu = 0.021503 (sec), leaf count = 38 \[ \left \{\left \{y(x)\to c_1 e^{-x/2}+c_2 e^{-x/2} x+c_3 e^{3 x}+e^x\right \}\right \} \]
Maple: cpu = 0.015 (sec), leaf count = 26 \[ \left \{ y \left ( x \right ) ={{\rm e}^{x}}+{\it \_C1}\,{{\rm e}^{3\,x} }+{\it \_C2}\,{{\rm e}^{-{\frac {x}{2}}}}+{\it \_C3}\,{{\rm e}^{-{ \frac {x}{2}}}}x \right \} \]