make them both fractions with the same bottom number
3-7/8 = 31/8
2-1/4 = 9/4 ( need to multiply by 2 to make it match ) so it becomes 18/8
now subtract 2 top numbers 31-18 = 13 so now you have 13/8
8/8=1 so 13-8 = 5 so the answer will be 1 and 5/8
