Yes, I understand the question. It is asking to identify the property of real numbers that justifies the equation:

$(3 + x) + y = 3 + (x + y)$

This equation uses the Associative Property of Addition. The associative property states that the grouping of numbers being added does not affect the sum. In this case, the parentheses are simply moved, but the terms and their sums remain the same. Specifically, the expression groups the terms differently without changing their order or sum.

Would you like more details on the associative property or have any other questions? Here are 5 related questions for further exploration:

1. How does the associative property differ from the commutative property?
2. Can the associative property apply to subtraction or multiplication as well?
3. What are some real-life examples of the associative property?
4. How can we visually represent the associative property in algebra?
5. How do the properties of addition and multiplication contribute to simplifying expressions?

Tip: Always pay attention to parentheses when working with the associative property—they indicate how terms are grouped.