First subtract whole numbers: 21-7=14. Now get fractions to one denominator: 2 and 4 go into 4 and 1/2=2/4. 1/4-1/2=1/4-2/4 which is negative, so borrow 1 from the difference of the whole numbers, making 14 into 13. Convert 1 into quarters: 4/4, and add to 1/4 to make 5/4. Now we can subtract 2/4 from 5/4 leaving 3/4. So the answer is 13 and 3/4.
To help you to understand the process imagine you have 21 pies plus a quarter piece: that's 21 1/4 pies. Now take away 7 pies leaving 14 pies and the quarter piece. You now have to remove 2 quarter pieces (half a pie) but you only have one quarter. So you cut one of the whole pies into quarters. Now you can take another quarter leaving 13 whole pies and three quarter pieces.