$y=-\sqrt{x^{2}-28}$
$y=\sqrt{x^{2}-28}$

$x=\sqrt{y^{2}+28}$
$x=-\sqrt{y^{2}+28}$

$y=\sqrt{x^{2}-28}$
$y=-\sqrt{x^{2}-28}\text{, }|x|\geq 2\sqrt{7}$