You have to find the common least common multiple for both denominators. For example, on number 2, The denominators are 3 and 4 the least common multiple of those numbers is 12 because 3 times 4 equals 12 and 4 times three equals 12. An easy way to find this is to list the multiple of both numbers until you find a match. Like this:
Multiples of 3: 3, 6, 9, 12, 15
Multiples of 4: 4, 8, 12, 16, 20
You can see that the smallest number they both have the same is 12. So the answer would be 12. Here are the answers for another one of the questions:
Number 3:
Multiples of 3: 3,6,9,12,18
Multiples of 6: 6,12,18
Common denominator: 6
