To find the least common denominator, you have to find the smallest number that both the numerator and the denominator will go into evenly. 

For example:

2 and 3 both go into 6

4 and 5 both go into 20

6 and 9 both go into 18

The next slide has a video about finding common denominators. If you are already comfortable with your level of mastery of this skill, you can skip the video. If not, it has a really great method to help you find the least common denominator

Copy & paste the link to watch it

https://www.youtube.com/watch?v=pZEmFSP3Z0I