\[ \boxed { \left ( xy \left ( x \right ) +a \right ) {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +by \left ( x \right ) =0} \]
Mathematica: cpu = 0.042505 (sec), leaf count = 40 \[ \text {Solve}\left [x=c_1 e^{-\frac {y(x)}{b}}-\frac {a e^{-\frac {y(x)}{b}} \text {Ei}\left (\frac {y(x)}{b}\right )}{b},y(x)\right ] \]
Maple: cpu = 0.047 (sec), leaf count = 30 \[ \left \{ {\it \_C1}+ \left ( -{{\rm e}^{{\frac {y \left ( x \right ) }{b} }}}bx+a{\it Ei} \left ( 1,-{\frac {y \left ( x \right ) }{b}} \right ) \right ) ^{-1}=0 \right \} \]