So the textbook uses a counter example to show this which is pretty simple. I tried playing around with the algebra. Ie.
$(A \cup B)-B$ is equal to $(A \cup B)\cap \bar{B}$ and associative law says this is equal to $A \cup (B\cap \bar{B})$. This results in $A \cup \emptyset$ which equals to $A$.
What am I doing wrong here?