\[ \boxed { \sqrt {{a}^{2}+{x}^{2}}{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +y \left ( x \right ) -\sqrt {{a}^{2}+{x}^{2}}+x=0} \]
Mathematica: cpu = 0.027504 (sec), leaf count = 57 \[ \left \{\left \{y(x)\to \frac {c_1}{\sqrt {a^2+x^2}+x}+\frac {a^2 \log \left (\sqrt {a^2+x^2}+x\right )}{\sqrt {a^2+x^2}+x}\right \}\right \} \]
Maple: cpu = 0.016 (sec), leaf count = 36 \[ \left \{ y \left ( x \right ) ={1 \left ( {a}^{2}\ln \left ( x+\sqrt {{a} ^{2}+{x}^{2}} \right ) +{\it \_C1} \right ) \left ( x+\sqrt {{a}^{2}+{x} ^{2}} \right ) ^{-1}} \right \} \]