When subtracting fractions, you should make sure they have the same denominator and turning them into an improper fraction. We should change 3 1/4 into a denominator of 8.
3 1/4 = 13/4
13/4 = 26/8
Now, let's subtract.
26/8 - 15/8
11/8
1 3/8
3 1/4 - 1 7/8 = 1 3/8