Final answer:
The missing property in the given expression is the commutative property of addition. This property states that the order of the numbers being added does not affect the result.
Step-by-step explanation:
Commutative property of addition
The missing property in the given expression is the commutative property of addition. This property states that the order of the numbers being added does not affect the result. In other words, changing the order of the numbers being added will not change the sum. In the given expression, 2s + 7 + 9 = 2s + 9 + 7 = 2s + 18.
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