Answer
The error is in step 5, where the Division Property of Equality was used to divide by $(b-a)$. Since it was given that $b=a$, we can calculate that $b-a=0$. Division by 0 is undefined, so this step of the proof is invalid.
Work Step by Step
Since division by 0 is undefined, you cannot include a step in the proof that would involve division by 0.
