If you look at the denominators with the goal of making them the same, you can see that 8 is a multiple of 4, meaning your job is easier than it could be. Multiply both the top and the bottom of -3/4 by 2 to make the bottom 8. You should get -6/8 while your 5/8 remains unchanged, meaning your final expression in 5/8 - 6/8.
