For dividing fractions, you flip the spots between the numerator and the denomintator of the second fraction. You then multiply this new fraction, a.k.a the reciprocal, with the first fraction you have.
7/3 is your second fraction. You flip the numerator with denominator to get 3/7.
Then you'll multiply by putting this new fraction into the equation.
21/4 x 3/7 = 63/28 which can be reduced into 9/4 by finding the greatest common factor between the numerator and the denominator.
So the answer is 9/4. :)