Answer:
D. Associative property of addition
Explanation:
The associative property of addition states "Changing the group of addends does not change the sum." This means it does not matter what order you use when adding the variables together, and PEMDAS does not apply.
The commentative property of addition states the order of the numbers when adding does not change the sum. While this is true for this equation, this is not the main focus as we are still adding the same way from lines 3-4 with 6+3, instead of 3+6