Does that make you less than half-good at life?
You know already that you need an exponential model. This is a good place to start. Do you know how to fit an exponantial model to a give data set?
One possible model is Hint: It's a least-squares fit. Can you do that?
After that, you find the value of ' ' such that .
Note: Quit beating yourself up. How can you not be good at it? Practice and GET good at it.