Approximating an exponential function with another function

Question

I have a function $f(\ell)=\exp\Big(-{\frac{a}{\ell b+c}}\Big)$. I would like to rewrite this function in the form $(1-d)g^{\ell}$.

How can I express $g$ and $d$ in terms of $a$, $b$ and $c$?

Here, a,b,c are constant real numbers.