Hi oldman,

I'd be happy to help you fit a trendline to your negative exponential data in Excel. Here are the steps you can follow:

1. First, enter your data into Excel. Make sure you have two columns: one for the x-values (usually time or some other independent variable) and one for the y-values (the dependent variable you want to model).
2. Select your data and create a scatter plot. To do this, go to the Insert tab and select Scatter from the Charts section. Choose the type of scatter plot you want (e.g. with or without markers).
3. Right-click on one of the data points in your chart and select Add Trendline. This will open the Format Trendline pane on the right side of the screen.
4. In the Format Trendline pane, select the Options tab. Under Trendline Name, choose Exponential.
5. Under Trendline Options, check the box for Display Equation on chart. This will show you the equation for the trendline.
6. If you want to adjust the appearance of the trendline, you can do so under the Line Style and Line Color sections of the Format Trendline pane.

That's it! Your chart should now have a trendline that fits your negative exponential data. If you want to make sure the trendline is a good fit, you can also add R-squared and/or adjusted R-squared values to your chart. To do this, go to the Chart Elements button on the right side of the chart and select More Options. Check the boxes for R-squared and/or adjusted R-squared.