Use a factor tree.
9
3,3
16
8,2
4,2,2
2,2,2,2
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7
Actually, 9 is NOT a prime number.
The answer is -16.
16 - 9 = 7.
Its square root is less than 9, but it is not evenly divisible by any number less than 9, other than 1, so it is a prime number