Let's break down the transformations step by step and identify the appropriate properties:

1. Line 1 to Line 2: $(2x + 9) + 5x \to (9 + 2x) + 5x$

   Here, the terms inside the parentheses $(2x + 9)$ are being rearranged as $(9 + 2x)$. This is an example of reordering the terms within an addition, which is the Commutative Property of Addition.

   Answer: Commutative Property of Addition

2. Line 2 to Line 3: $(9 + 2x) + 5x \to 9 + (2x + 5x)$

   In this step, the grouping of terms has changed. Initially, $9$ was grouped with $2x$, but now $2x$ is grouped with $5x$. This change in grouping reflects the Associative Property of Addition.

   Answer: Associative Property of Addition

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Would you like any further clarification on these properties?

Here are some follow-up questions that could expand your understanding:

1. What is the difference between the commutative and associative properties?
2. How would you apply the commutative property to a multiplication problem?
3. Can the associative property be applied to subtraction or division?
4. Why is it important to recognize these properties in solving algebraic expressions?
5. How does the distributive property differ from the commutative property?

Tip: Understanding properties like the commutative and associative properties can help simplify complex algebraic expressions efficiently!