The denominator must be the same, so just think of the LEAST COMMON MULTIPLE, The LCM here is 8, because you can multiply 4 by 2 to get 8.
so when you multiply, you multiply both on the top and on the bottom, so this would be:
3 x 2 / 4 x 2 = 6 / 8
Now you can add or subtract: 6/8 - 1/8 = 5/8