Cannot get expfit to work

[pchristensen-2]

I have a set of data (x and y coordinates) from excel (see top of attached file), and when excel didn't want to give me the function for the data, I had to use a website to get a working onw. I am now trying to work my way from the data to the function in Mathcad 15, but I cannot get expfit to work. The guess values is where it goes wrong (probably just confusing me).

I have the working function and coefficients at the bottom of the attached file, everything else is trial and error. Can somebody help getting those coefficients from expfit?

Thanks in advance!

[Werner_E · Accepted solution]

Mathcad is quite sensible with respect to the guess values.

The second element in the vector sure has to be negative:

[pchristensen-2]

Thanks Werner_E, it feels bad when the solution is so simple, but math never was my forte. But the function provides good values now!