The LCM of any set of numbers that has a prime or a number of primes in common. The way to find it is to factor the given number down to it's primes.
3: 3
9: 3 * 3
15: 3 * 5
The number which has been bolded is the LCM in this case 3 <<<< Answer
This is not a particularly good question to learn what it might be Try these 3 numbers.
42: 2*3 * 7
84: 2 * 2 * 3 * 7
210: 2*5 * 3 * 7
The LCM for this bunch is 2 * 3 * 7 = 42
