Alright - we want to start off by seeing if this number would be positive or negative. Debit means that you owe something, or that you have to pay that amount of money. If you had money to buy things with, that would be positive, but since debit means you haven’t paid something off yet, that is negative. Next, since you have 40 dollars in debt, you have -40 dollars.
Feel free to ask further questions!