Frequently, and even historically, the creation and use of negative numbers has been associated with debts. To denote a certain amount of debt, we add a negative sign before the amount to indicate the fact that it's a debt.
Here, Gena's food cart is $225 in debt, so we could indicate that with the negative number -225. If we're tripling the debt, we're going to be multiplying that debt by 3, which corresponds to the expression -225 x 3. If you're adding onto your debt, you're naturally going to still be in debt, so we leave the negative sign on and multiply 225 and 3, obtaining -225 x 3 = -675. Gena's food cart is now $675 in debt.