The lowest common multiple of 9 and 12 is 36. The least common multiple of two numbers (x and y) is the smallest number that has both x and y as factors.
The best method to find the least common multiple is to split both numbers into prime factors. That means 9 is 3 x 3, and 12 is 2 x 2 x 3. Next, it is best to count how many times each prime factor appears in each number. Three appears twice in 9 and once in 12. Two appears twice in 12 and never in 9. Take the maximum amount each prime appears in either number. That means 3 twice and 2 twice. Finally multiply the primes together to find the least common multiple. 3 x 3 x 2 x 2 equals 36, which is the least common multiple of 9 and 12.