\[ y''(x)=-\frac {y(x)}{(a x+b)^4} \] ✓ Mathematica : cpu = 0.176026 (sec), leaf count = 78
\[\left \{\left \{y(x)\to c_1 e^{\frac {\sqrt {-\frac {1}{a^4}} a}{a x+b}} (a x+b)+\frac {c_2 e^{-\frac {\sqrt {-\frac {1}{a^4}} a}{a x+b}} (a x+b)}{2 \sqrt {-\frac {1}{a^4}} a^2}\right \}\right \}\]
✓ Maple : cpu = 0.069 (sec), leaf count = 43
\[ \left \{ y \left ( x \right ) ={\it \_C1}\, \left ( ax+b \right ) \sin \left ( {\frac {1}{a \left ( ax+b \right ) }} \right ) +{\it \_C2}\, \left ( ax+b \right ) \cos \left ( {\frac {1}{a \left ( ax+b \right ) }} \right ) \right \} \]