How to use InverseFunction?

Questions
1

Hi, I am new to Mathematica. I would like to find the inverse function of the following function f(x):

80691.jpg

I have tried to do it using InverseFunction:

40352.jpg

3.jpg

However, Mathematica couldn’t seem to provide me an answer.
Any help is much appreciated.

2

Check out the syntax for Function. I would try with Solve:

With[{k1 = 5, b = 2}, 
 Solve[y == 
   1 - Sum[a^k/k! Exp[-a], {k, 0, Infinity}] Sum[
      1/n! Exp[b x] (b x)^n, {n, 0, k1}], x]]

but it seems too hard.