2) 7+ (1 + 9) = (7 + 1) +9
Property:

Associative Property

Step-by-step explanation:

In your equation, you're adding three digits together.

When you're adding or multiplying, the order in which you it doesn't matter:

ex.  1 + 3 + 5  =  3 + 5 + 1   <- This is the commutative property (not associative)

Adding parentheses indicated the order in which you do an operation, but when adding, order doesn't matter, so you're able to move around the parentheses. When you move around the parentheses, that's an example of the associative property.

It's important to not confuse the associative property with the commutative property.

The associative property is moving around parentheses

While the commutative property is moving around the numbers.