\[
\boxed{
{\frac {{\rm d}^{3}}{{\rm d}{x}^{3}}}y \left( x \right) +f \left( x \right) {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right) +g \left( x \right) {\frac {\rm d}{{\rm d}x}}y \left( x \right) + \left( f \left( x \right) g \left( x \right) +{\frac {\rm d}{{\rm d}x}}g \left( x \right)  \right) y \left( x \right) =0}
\]