$y=-\sqrt{x^{2}-256}$
$y=\sqrt{x^{2}-256}$

$x=\sqrt{y^{2}+256}$
$x=-\sqrt{y^{2}+256}$

$y=\sqrt{x^{2}-256}$
$y=-\sqrt{x^{2}-256}\text{, }|x|\geq 16$