The property demonstrated in the example 2 + (5 + 6) = (2 + 5) + 6 is The Associative Property.
We can tell this is the associative property because of what the associative property states. The associative property states that it doesn't matter where you put parenthesis when adding or multiplying numbers. Since we see an equation that has addition, and two equations being equal to each other, despite having different placements of parenthesis, we can see that this equation is demonstrating The Associative Property.