Common divisors are 2 and 3. Common multiples are found by making a table:
2 x 12 = 24
3 x 12 = 36
2 x 18 = 36 <-- ANSWER
You can also look at prime divisors:
12 - 1, 2, 2, 3
18 - 1, 2, 3, 3
So if you go three times the first one and 2 times the second one, you get 36.
