The issue is that, in order to determine the line of best fit, you need to know linear algebra, I don't know if you have seen that. The concept of Linear Regression (https://en.wikipedia.org/wiki/Linear_regression) requires minimizing a problem in linear algebra, with matrices. Have you seen any of that?
Other than that, the only other way I know how to do is by plotting it on a program like Excel or using a graphing calculator. Here, I used Excel