You need to make the fractions into improper fractions with the same denometer. 3 4/7 to an improper fraction is 25/7 and 3 1/4 is 13/4. Now get common denometers, in going to choose 28. 100/28 is the same as 25/7 and 91/28 is the same as 13/4. Now subtract 100/28 and 91/28 and you get 9/28. So you need to add 9/28 of a cup.