Geogebra unable to solve equation with pre-defined function Disclaimer
The issue seems to be due to restricting the function on an interval, even though the solution is within the given range. It seems to be a bug in Geogebra, and I have submitted a ticket for the developers.
Leaving this question un-deleted as documentation.

Background
I have a function $f(x)$ defined by the FitExp command, restricted by $0 \leq x \leq 50$.
I want to solve $f(x) = 100$, and I've tried both the Solve and NSolve commands, but they both yield $?$ as the answer. See image below.
Question
Is there another command I should be using for this, or is there some other input needed before this will give the answer?
NOTE
All commands in CAS were entered after all the commands in the Algebra view. Meaning, $f(x)$ was defined before I tried to use Solve and NSolve.


Im using the desktop version of geogebra. Defining your function in the algebraic view
$$f(x):=6.17 e^{0.1 x}$$
by typing in the bar
f(x)=6.17 e^(0,1 x)

and after using the command in the CAS view
Solve[f(x)=100,x]

I get the answer
$$\left\{x=10\ln\left(\frac{10000}{617}\right)\right\}$$
Indeed it also works with the command
Solve[f(x)=100]

Probably it doesnt work for you because you had restricted $f$ in $[0,50]$ (I dont know how to do this, but I can see it in your picture).
