Factor the Expression completely$ (a+b)^2 - (a-b)^2$ I don't understand this question. The answer in the book is $4ab$, but how is that term a factor?
I was thinking along the line that this was a difference of squares example. $a^2-b^2 = (a+b)(a-b)$
My answer is $[(a+b)-(a-b)][(a+b)+(a-b)]$
What do I not understand?  

Note that you don't even need to factor it, just expand the expression.
$$(a+b)^2-(a-b)^2$$
$$=(a^2+b^2+2ab)-(a^2+b^2-2ab)$$
$$=a^2+b^2+2ab-a^2-b^2+2ab$$
$$=4ab$$
If you have to factor it, remember the difference of squares $a^2-b^2=(a+b)(a-b)$. In this case, $a^2$ is $(a+b)^2$, and $b^2$ is $(a-b)^2$.
$$(a+b)^2-(a-b)^2$$
$$=(a+b+a-b)(a+b-(a-b))$$
$$=(2a)(a+b-a+b)$$
$$=(2a)(2b)$$
$$=4ab$$
