Can anyone help me to get $\displaystyle S(x)$ from

$\displaystyle

S^{'} (x)=x^{-b} e^{ -a x}

$

$\displaystyle x>0$

If I tryed to use integration by parts, by I can't figure put, what to call $\displaystyle f$ and $\displaystyle g$. And Maple does not give me anything to go with.