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