Think of it as a number line. You start at 0, then go to -3. Then you count from -3 to 10 how many yards you would need to gain in order to get to 10 yards. It's really just 10+3, which is 13. You would need to gain 13 yards.
I think the answer is 13. A loss of 3 yards would give you -3 yards gained. So to get to 10 yards gained you would first have to gain back 3 yards and then the 10 you originally wanted.
