Final answer:
The common denominator of 12 and 9 is 36.
Step-by-step explanation:
To find a common denominator between 12 and 9, we need to find the least common multiple (LCM) of these two numbers. The factors of 12 are 1, 2, 3, 4, 6, and 12, while the factors of 9 are 1, 3, and 9. The LCM is the smallest number that can be divided evenly by both 12 and 9, which in this case is 36. Therefore, 36 is a common denominator for both denominators.