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