Answer: 9
Explanation:
Find the factors (numbers that the given number can be divided by to make a whole number) of each number:
9: 1,3,9
18: 1,2,3,6,9,18
It's quite easy here because the numbers are small but if you get bigger numbers, it might be helpful to draw out a prime factorization tree. And then multiply together all the common prime numbers. (I haven't explained this in great detail but if you want to know leave a comment :))