\[ (y(x)+x)^2 y'(x)-a^2=0 \] ✓ Mathematica : cpu = 107.166 (sec), leaf count = 20
\[\text {Solve}\left [a \tan ^{-1}\left (\frac {y(x)+x}{a}\right )+c_1=y(x),y(x)\right ]\]
✓ Maple : cpu = 0.054 (sec), leaf count = 24
\[ \left \{ y \left ( x \right ) =a{\it RootOf} \left ( \tan \left ( {\it \_Z} \right ) a-{\it \_Z}\,a+{\it \_C1}-x \right ) -{\it \_C1} \right \} \]