To complete the proof using the Commutative Property, we need to rearrange the terms of the left-hand expression.
The correct missing step is:
Step 1: 10 + 12a + a = 10 + 13a
So the sequence of steps would 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.
Thus, the final answer you are looking for is:
10 + 12a + a = 10 + 13a.