Problem with finding a time symbolic expression at which function become unity

Ok, I see, ok, thanks.

Now going also to an numerical approach solution for this problem. How to find t2 value above obtained by symbolic means way that gives f(t2(), , )=1?
For example one method at which I am thinking by now (but also other approaches are welcome): to iterate time t and then to look/check where f() have the desire value, in this case 1.
How to implement below things?

For this we need to check in the f() where f() = 1:

Then:

And finally to find the corresponding index found at Step 2 in the t column vector.

But as I said, other methods to achieve this thing (of finding time t at which f() is equal with 1) are welcome if exists. This above method is only at which I am thinking by now.