\[ \boxed { \left ( -{a}^{2}+{x}^{2} \right ) \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{3}+bx \left ( -{a}^{2}+{x}^{2} \right ) \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{2}+{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +bx=0} \]
Mathematica: cpu = 0.023503 (sec), leaf count = 86 \[ \left \{\left \{y(x)\to c_1-\frac {b x^2}{2}\right \},\left \{y(x)\to c_1-\tan ^{-1}\left (\frac {x \sqrt {a^2-x^2}}{x^2-a^2}\right )\right \},\left \{y(x)\to \tan ^{-1}\left (\frac {x \sqrt {a^2-x^2}}{x^2-a^2}\right )+c_1\right \}\right \} \]
Maple: cpu = 0.016 (sec), leaf count = 52 \[ \left \{ y \left ( x \right ) =-{\frac {b{x}^{2}}{2}}+{\it \_C1},y \left ( x \right ) =-\arctan \left ( {x{\frac {1}{\sqrt {{a}^{2}-{x}^{2} }}}} \right ) +{\it \_C1},y \left ( x \right ) =\arctan \left ( {x{\frac { 1}{\sqrt {{a}^{2}-{x}^{2}}}}} \right ) +{\it \_C1} \right \} \]