\[ \boxed { \left ( y \left ( x \right ) +x \right ) ^{2}{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) -{a}^{2}=0} \]
Mathematica: cpu = 0.036005 (sec), leaf count = 21 \[ \text {Solve}\left [y(x)-a \tan ^{-1}\left (\frac {y(x)+x}{a}\right )=c_1,y(x)\right ] \]
Maple: cpu = 0.047 (sec), leaf count = 24 \[ \left \{ y \left ( x \right ) =a{\it RootOf} \left ( \tan \left ( {\it \_Z } \right ) a-{\it \_Z}\,a+{\it \_C1}-x \right ) -{\it \_C1} \right \} \]