Michelle has $500 to spend on a bicycle store all prices below include tax

You can consider lost yards as negative, and gaines yards as positive. So, if they lost 9 yards and then gained 9 yards, the expression is

-9+9=0

This means that the team is exactly where it started (which maked sense, because they lost exactly as much as they gained.

If they gained first and lost then, it wouldn't change a thing: you'd have

9-9=0

and so the team is always where it started.