What is the common denomitator of this fractions 9/14 1/7 8/21

So first find the factors of the denomenatores

14=2 times 7

7=7

21=7 times 3

common one would be having at least one 2, one 7, and one 3

find what numbers can be made by all of them

2 times 7 times 3

2 times 7

7

7 times 3

they are all in it

2 times 3 time s7=42

common denoomenator is 42

21 would be the answer because you can multiply everyone of those numbers to get 21.