I'm assuming you mean 17- (-7) = 24. It equals this because you are taking away negative, which means more positive. For example: Say you have $24, and you owe 7 dolars to a friend. If you didn't pay the 7 dollars, it would be negative $7 on your account because eventually you would have to pay it. That would be $17. But if you did your friend a favor, and he chose to take away your debt, you would have minused -7 dollars from your account, causing you to have 24 dollars!
+165 