Feedback for a Line of Best Fit (in point slope form)


I am working on an activity about modeling data.

Here is what I have so far: Snowy Tree Crickets • Activity Builder by Desmos

My issue with the feedback on screen 3. I currently have it defined based on the slope and y-intercept of the line they enter, but depending on the point they choose for their equation, the y-intercept can seem good but the line is actually too far above or below the data.

I realized it would be better to define a “good” line based on the residual values for a point at each end of the data set, but here is where I got stuck. I want to determine the values of f(50) and f(85) for the equation the student inputs and compare that to the values for the regression equation for the data, but I don’t know how to get the values from the student equations. It feels like something I would do in the graph, but I’m not really sure that is right.

FYI- I’m pretty new to CL, but it is one of my goals for this year to learn more and push myself to get better with it. I’m getting there slowly, but I could use some big help on this one!