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