\[
\boxed{
{\frac {{\rm d}^{3}}{{\rm d}{x}^{3}}}y \left( x \right) -3\,a{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right) +3\,{a}^{2}{\frac {\rm d}{{\rm d}x}}y \left( x \right) -{a}^{3}y \left( x \right) -{{\rm e}^{ax}}=0}
\]