You can set up a proportion that looks like 3/7 = x/21. X being the number that you are trying to find. Seven goes into 21 three times. So multiply 3 also by that 3 to get 9. So three sevenths of twenty-one is 9.
The question is asking what the original number is. So its not 9. If you do a inverse operation which is 2 divided by 3/7 you get 49. To check your work if you multiply 49 by 3/7 you get 21.
