Find the difference of 3/4 + -1 1/4 on a number line

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

The answer is: a = about 63.59ft2 for 15