Name the property that is shown by the equation (3 + 9) + 11 = 3 + (9 + 11)

The associative property is being demonstrated. This is because the order of the numbers themselves is not being changed, but the groupings are. The associative property is often confused with the commutative property. In the commutative property, the order of the numbers are changed. Example -

3 + 4 + 7 = 7 + 3 + 4

In this problem, the numbers' order is changed, demonstrating the commutative property.

However, in the problem that you wish to solve, the numbers' order is not changed, but the parentheses around them is changed (also known as the groupings), demonstrating the associative property.