\[ (y(x)+x)^2 y'(x)-a^2=0 \] ✓ Mathematica : cpu = 0.139244 (sec), leaf count = 21
DSolve[-a^2 + (x + y[x])^2*Derivative[1][y][x] == 0,y[x],x]
\[\text {Solve}\left [y(x)-a \tan ^{-1}\left (\frac {y(x)+x}{a}\right )=c_1,y(x)\right ]\] ✓ Maple : cpu = 0.063 (sec), leaf count = 24
dsolve((y(x)+x)^2*diff(y(x),x)-a^2 = 0,y(x))
\[y \left (x \right ) = a \RootOf \left (\tan \left (\textit {\_Z} \right ) a -\textit {\_Z} a +c_{1}-x \right )-c_{1}\]