The range of the y=9−x2 is
9,∞
(9,∞)
0,3
(3,∞)
9−x2
Least value of square root is 0 when 9−x2=0 or x=±3
Also, greatest value of 9−x2 is 9 when x=0 . Hence we have 0≤9−x2≤9⇒9−x2∈[0,3]