Final answer:
Bill used the commutative property to find the actual December temperature.
Step-by-step explanation:
Bill used the commutative property to find the actual December temperature. The commutative property states that the order of addition does not affect the sum. This means that whether you add -5 + 3 or 3 + -5, you will get the same result, which is -2 in this case.
Learn more about Commutative property in addition