To determine the missing step using the Commutative Property, we want to rearrange the terms in the expression \(12a + 10 + a\) to match the form presented in the second step, \(10 + 13a\).
Given this, the missing step can be:
Step 1: \(10 + 12a + a = 10 + 13a\)
This makes use of the Commutative Property, which states that the order of addition does not change the sum.
So, the complete sequence will be:
Step 1: \(10 + 12a + a = 10 + 13a\)
Step 2: \(10 + 13a = 10 + 13a\)
Step 3: The expressions are equivalent because they both equal the same value.
Therefore, the correct response is:
10 + 12a + a = 10 + 13a.