The line of best fit is a line that follows the direction of the dots. It doesn't have to be perfect, it's just a general line to represent the data. Sometimes there isn't a line of best fit (if there is no correlation). It normally touches most of the dots.
The slope is about 1/2 and the point where the line touches the y axis is at 40 so that means an equation could be y = 1/2x + 40
When the advertising expenditure (x value) is 18,000, substitute it into the equation above. y = 1/2(18,000) + 40 which equals y = 9,040 . This is the expected revenue.
Hope this helps!
