They’ve started out with 0yds of course so, -8yds on the first down. They’ve gained 20yds overall in both downs. You’re now trying to find “x”. -8+x=20. To find “x”, you do 20+8 because the 8 is negative and you would lose numbers in order to get 20, so “x” equals 28. Therefore, you’ve gained 28 yds in the second down and lost 8 yds in the first down.
I really, really hoped that helped. It took awhile to figure out, but I’m sure I’ve got it now.
