get a common denominator which is 24
3 7/8 = 3 21/24
6 /4 = 6 6 /24
3 21/24 - 6 6 /24
big number minus small number take sign of larger
6 6/24
- 3 21/24
-------------
5 30/24
3 21/24
--------------
2 9/24
2 3/8
the sign of the larger is negative
-2 3/8
