answer:You can think about it as the same reason why saying “She didn’t not buy 5 apples” means that she bought 5 apples. If you lost $5 on a stock and your losses double, that means you lost $10 (-5×2). If your losses negative-double, that means you gained $10 (-5×-2). If this explanation doesn’t make sense, you can read some more (as well as a proof) by Dr. Math.