Answer:
18
Explanation:
We need to find the LCM (lowest common multiple) of 3, 6, and 9. Let's count by multiples of 3 to find it.
3 (not a multiple of 6 or 9), 6 (not a multiple of 9), 9 (not a multiple of 6), 12 (not a multiple of 9), 15 (not a multiple of 6 or 9), 18.
Since 18 is the first number that is a multiple of 3, 6, and 9, that is the answer.