Simplify: 9 minus negative 15.

1. Subtracting a negative number is the same as adding the opposite of that negative number, which is positive. 2. 9 minus negative 15 becomes 9 plus 15. 3. 9 plus 15 equals 24. 4. Treating the two negative-looking signs as canceling to a plain subtraction gives 6, which drops one of the signs instead of converting subtraction into addition. _Source: OpenStax Prealgebra (CC BY 4.0), Ch 3 "Integers", section 3.3 Subtract Integers_