So we want to know why is it necessary to rename 4 1/4 if we subtract 3/4 from it. First, lets turn 4 1/4 into a fraction: 4 1/4 = 17/4. Now lets subtract 3/4 from it: 17/4 - 3/4 = 14/4. So 14/4 becomes 3 2/4. And now we see why is it necessary to rename 4 1/4.
