The property used to rewrite the expression 9x + y as 7y + 9x is the Commutative Property of Addition.
The property that was used to rewrite the expression 9x + y as 7y + 9x is the Commutative Property of Addition. According to this property, the order of the terms can be changed without affecting the sum. So, in this case, the terms "9x" and "y" were rearranged to "y" and "9x".
