Deriving $\frac{\partial VaR_\eta}{\partial \psi}$?

Question

For all $s\in N, \lambda, l>0, \psi\in(0,l), \eta\in(0,1)$, $g$ is calculated from the following equation:

$\sum_{k=0}^{s-1} \frac{\lambda(l-\psi-g)^k}{k!}e^{-\lambda(l-\psi-g)}=1-\eta$

How can we derive $\frac{\partial g}{\partial \psi}=?$