We have the points
(1,8)
(2,6)
(3, 7)
(4,5)
(5,3)
(6,4)
(7,2)
(8,1)
(9,3)
There is a formula for this but it's a little involved !!!
Let's use some technology to find the best ft line : http://www.learningaboutelectronics.com/Articles/Regression-line-calculator.php#answer
The line is y = -.77x + 8.2