Answer :
No, because there is no number by which both 21 and 8 can be divided exactly :)
When you're looking to simplfy a number, the best thing to do is to list all of the factors of that number;
21- 1,3,7
8- 1,2,4,8
The only common factor between these is one, and this is what they're currently in, therefore, it cannot be simplified.
Hope this helps :)