\[ \boxed { a\cos \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) +b{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +x=0} \]
Mathematica: cpu = 0.011001 (sec), leaf count = 21 \[ \text {DSolve}\left [a \cos \left (y'(x)\right )+b y'(x)+x=0,y(x),x\right ] \]
Maple: cpu = 0.031 (sec), leaf count = 18 \[ \left \{ y \left ( x \right ) =\int \!{\it RootOf} \left ( a\cos \left ( { \it \_Z} \right ) +{\it \_Z}\,b+x \right ) \,{\rm d}x+{\it \_C1} \right \} \]