Subtraction is the opposite of addition. If adding a positive integer results in a greater number, then subtracting a positive integer results in a lesser number. If adding a negative integer results in a lesser number, then subtracting a negative number results in a greater number.
Subtracting a negative is the only thing today that is new. We want students to conceptualize that taking away a negative, debt for instance, results in a greater number. We will introduce a rule for when we see 7 - - 9. “Add the opposite.”