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lauren246
The answer is 9. Put 21 over one, cross cancel and then multiply straight across. Hope this helps!
Ok so first to set up the equation, you would do 3/7×21/1. 
(3)(21)
---------
(7)(1)
So 3×21 is 63 and 7×1 is 7.
63/7 is simplified down to 9/1
So your answer is 9. Hope I helped!