[tex]y=x^2-18\ \ \ \ |\text{add 18 to both sides}\\\\y+18=x^2\ \ \ \ |\text{square roth of both sides}\\\\\sqrt{x^2}=\sqrt{y+18}\to|x|=\sqrt{y+18}\\\\x=\pm\sqrt{y+18}\\\\Answer:\ A.\ x=\pm\sqrt{y+18}[/tex]
