Answer:
It's 21.
Step-by-step explanation:
The factors of 9 are 3 and 3. (You need to multiply 3x3 to get 9.) The factors of 10 are 5 and 2. But, if you multiply 3 by 5, which gives you 15, and multiply the other 3 by 2, which gives you 6, and then add 6 to 15, you get 21.
