(u + 7) + 13 = u + (7 + 13) The associative property of addition says that you can add numbers regardless of how they're grouped and get the same result. So let's look at the options and see what makes sense. (u + 7) + 13 = u + (7 + 13) * This is showing the associative property like the definition states. So this is the correct choice. 21 + 2y = 2y + 21 * This is a demonstration of the commutative property of addition. So it's the wrong choice for this question. 3â‹…(5â‹…x)=(3â‹…5)â‹…x * This is showing the associative property of multiplication. So it's the wrong choice for this question. If m + 3 = 18, then 18 = m + 3. * This is a demonstration of the symmetric property of equality. So for this question, it's the wrong choice.