You and your friend go to a mexican restaurant. you order 2 tacos and 2 enchiladas. your friend orders 3 tacos and 1 enchilada. your bill was $4.80. your friend’s bill was $4.00. what was the price of an enchilada?

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.