Well, in order to simplify it, you need to see if a number goes into both 3 and 21.
This number is 3 because it does go into 3 and 21.
3 goes into 3 once
3 goes into 21 7 times.
So, from what you know now, it should be easy to simplify it furthur. However, I will still give you the correct simplified answer.
3/21 = 1/7
And 1/7 is your final answer. I hope all of this helps you!!! :)