\[
\boxed{
{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right) =-2\,{\frac {n\cosh \left( x \right) {\frac {\rm d}{{\rm d}x}}y \left( x \right) }{\sinh \left( x \right) }}- \left( -{a}^{2}+{n}^{2} \right) y \left( x \right) =0}
\]