28.
Your starting number is x. After you divide it by 4, the difference between x and that new number is 21. When you write that as an equation, it looks like this:
x - ( x / 4 ) = 21
Then solve for x.
( 4x / 4 ) - ( x / 4 ) = 84 / 4
4x - x = 84
3x = 84
x = 28