Exactly what was predicted by the chart trendline and LINEST. When we click “Solve”, the Solver does its thing and finds that the values m = 165.36 and b = -79.85 define the best-fit line through the data. When properly set up, the solver dialog should look like this:
That means we can use them dynamically in a calculation somewhere else in the spreadsheet.
This was obviously more work than using a trendline, but the real advantage here is that the slope and y-intercept values have been output to a cell.