\[ (y(x)+x)^2 y'(x)-a^2=0 \] ✓ Mathematica : cpu = 0.121874 (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 = 0.046 (sec), leaf count = 24
\[ \left \{ y \left ( x \right ) =a{\it RootOf} \left ( \tan \left ( {\it \_Z} \right ) a-{\it \_Z}\,a+{\it \_C1}-x \right ) -{\it \_C1} \right \} \]