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The commutative property of addition is where we can add two numbers in any order we want. Example: 2+3 = 5 and 3+2 = 5.
In general, the rule is x+y = y+x.
In our case, 2x and 15 can be added in any order. So that's why 2x+15 is the same as 15+2x.
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Side note: a similar rule applies for multiplication as well. This is called the commutative property of multiplication. Example: 9*3 = 27 and 3*9 = 27. So we can say x*y = y*x.
