To determine the missing step in proving the equivalence of \( 12a + 10 + a = 10 + a + 12a \) using the Commutative Property, we can identify that the missing step involves rearranging the terms on the left side of the equation to match the right side.
Here's the completed proof with the missing step included:
Step 1: 12a + 10 + a = 10 + 12a + a (Rearranging the terms using the Commutative Property)
Step 2: 10 + 12a + a = 10 + 13a (Combining like terms)
Step 3: The expressions are equivalent because they both equal the same value.
So, the missing step is:
Step 1: 12a + 10 + a = 10 + 12a + a
This step utilizes the Commutative Property of addition, allowing us to rearrange the terms freely.