Example: If you owe someone $7, and you give them $2, how much do you still owe? You are owing him $7 so it's a negative and you give him $2 which is a negative as well since you are giving it to him. So, to answer how much do you still owe, you would add $2 to the $7 you owed, so now, you owe him $5, which is a negative since you OWE him.
