I can't understand how did they solve the second part in this attached image. The part where (xi-(average of x))(yi-(average of y)) becomes xi (yi-(average of y)) then switches and finally we get the end general form. Can you please explain the process? I understand A.7 but I don't get A.8. I cannot find the rules they used for this. A.7 is pretty straightforward. Thank you so much!