Obtaining The Equation Of Line Of Best Fit
If we have a straight line then we can calculate an equation for this line from the graph and then use this equation as a formula.
We need two things: the gradient of the line and the y-intercept, or we can work out the equation from two points on the line.
For the gradient we need to draw a right angled triangle onto the line of best fit and then work out a vertical side distance and a horizontal side distance. The gradient is the vertical distance divided by the horizontal distance.
The y-intercept is where the line of best fit (extended if needed) cuts through the vertical axis.
The material on graphs gives the details.