In your shoes, I'd write, "Find the difference between 3/4 and -1 1/4."
3/4, being positive, is greater than -1 1/4. But the magnitude of -1 1/4 is greater than the magnitude of 3/4, so your result (the difference) will take the " - " sign.
3-5
The problem then looks like 3/4 - 5/4 = ---------- = -2/4 = -1/2
4