The easiest way to do this, is to use the Fit Y by X platform, and have it draw your curve and then to select
- Fit Each Value
- Fit Line
- Select Local Data Filter and select the X value to be filtered
Here are the results with my example data
Note that under the Parameter Estimates you can see the slope of .0038645
Now all you have to do, is to use the Data Filter to select only the values on the left side of the graph.
The regression now is only calculated on the down slope, which is -0.490244
Jim