Think of it like a debt being taken away. When you subtract a negative, you're removing a shortage, so you gain. It’s +9 because -(-9) is like cancelling a loss.
But on the number line, moving right for gain only works if you have something to start with. Our family’s store is at zero some months, and if I subtract the debt to the wholesaler, I’m still not right of zero—I’m just at the door, borem, not owing. The line shows theory, but my ledger shows a different truth.
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