$\displaystyle

x^4 - y^4 = (x^2 + y^2)(x^2 - y^2)

\\ = (x^2 + y^2)(x + y)(x - y)

$

I'm ok with this until the last part. It looks like as the second term is a difference of squares it has been factored as well. I'm just a bit fuzzy on the use of brackets here. Can anyone help?

Thanks.