The answer you are looking for is associative property of addition.
In associative property, no matter where the parenthesis are, a + (b + c) is going to be equal to (a + b) + c. Since the parenthesis follow the equation a + (b + c) = (a + b) + c, you can assume you are using the associative property. Lastly, add "of addition" to associative property, since you are using addition, not multiplication.
