3/4-9/2=3/4-18/4=-15/4. You can't simply subtract numerators and denominators.
You need to get a common denominator for the fractions, so we write 9/2 as 18/4 by multiplying top and bottom by 2. That gives us a simple sum that says subtract 18 quarters from 3 quarters. Think of quarters as coins if it helps. You could also think of the quarters as objects like apples: 3 apples minus 18 apples. The answer is -15 apples. So that is -15 quarters or -15/4.