Formula to create a regression line chart.

I have NO idea of how to do this so I hope you guys might be able to offer some assistance.

I have attached an example showing the “Days” (days eating wheat) from 120 to 180 days.

As far as column B goes- this is the Average Daily Gain. I can tell you that at 120 days the gain is 2.80 and at 180 its 2.25. Just so you understand what I am actually looking at this is Cattle. So as you can imagine as the Cattle get larger they daily gain will shrink but its not lateral (a straight line). The slope should start out slight then drop off almost exponentially as you approach the 180th day to 2.25.

I assume there needs to be a couple of additional numbers to make this happen but again I don’t even know where to start.

Thank you for your help.