for A: Do -9+9. It is negative because they LOST yards, which means backwards momentum xDD
For B: They are right where they started. -9+9=0. They lost 9, then gained the 9 they had lost.
C: It would not have been different. No matter what way you add it, it would have been the same thing. Instead it would be 9-9, which is still 0.
Hope this helps!