$\text { Find the absolute maximum value of } y = \sqrt { 9 - x ^ { 2 } } \text { on the interval } [ - 9,9 ] \text {. }$

$\text { Find the absolute maximum value of } y = \sqrt { 9 - x ^ { 2 } } \text { on the interval } [ - 9,9 ] \text {. }$