Final answer:
The expression 21 + 9 can be rewritten as 9 + 21 using the commutative property of addition.
Step-by-step explanation:
The given expression is 21 + 9. Using the commutative property of addition, we can rearrange the order of the numbers without changing the sum. So, 21 + 9 is equal to 9 + 21. Therefore, the expression can be rewritten as 9 + 21.
Learn more about Commutative property of addition