\[
\boxed{
{\it d4y} \left( x \right) -2\,{a}^{2}{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right) +{a}^{4}y \left( x \right) -\lambda\, \left( ax-b \right)  \left( {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right) -{a}^{2}y \left( x \right)  \right) =0}
\]