Simplify the algebraic expression completely. b24a2−4ab+b2+a−b2a−b

To solve this problem you must apply the proccedure shown below:
 1. You have the following expresssion given in the problem above: b^24a^2−4ab+b^2+a−b^2a−b 
 2. Therefore, you must factor the common terms to simplify the expression above, as following:
 ba(b^23-b-4)+b(b-1)+a
 Then, as you can see, the answer is: ba(b^23-b-4)+b(b-1)+a

What mathematical statement is missing ? ? the answer is b