\[ -11 y'(x)-8 y''(x)+4 y^{(3)}(x)-3 y(x)+18 e^x=0 \] ✓ Mathematica : cpu = 0.0244796 (sec), leaf count = 38
\[\left \{\left \{y(x)\to e^x+c_1 e^{-x/2}+c_2 e^{-x/2} x+c_3 e^{3 x}\right \}\right \}\] ✓ Maple : cpu = 0.017 (sec), leaf count = 23
\[ \left \{ y \left ( x \right ) = \left ( {\it \_C3}\,x+{\it \_C2} \right ) {{\rm e}^{-{\frac {x}{2}}}}+{\it \_C1}\,{{\rm e}^{3\,x}}+{{\rm e}^{x}} \right \} \]