To me it looks like finding the root of a function:
Z(x) = 0,
where Z(x) = f1(x) - f2(x)
My approach in a spreadsheet would be:
Select an interval [x1 , x2] where you have a positive and a negative value of Z(x)
Z(x1) * Z(x2) < 0
Narrow your interval until Z(x) = 0 or sufficient small.
There are some other methods but I never used them in spreadsheets, just plain programming in pascal or fortran, but it must be possible. Check out a good book about numerical methods.
Regards Steven van Els
SAvanEls@cq-link.sr