The graph shows the height h, in feet, of s football at time t, in seconds, from the moment it was kicked at ground level.
We are supposed to find the average rate of change in height from t=1.5 seconds to t=1.75 seconds.
Average rate of change in height=

As we can see from the the change in distance for the time interval 1.5 second to 1.75 second=

and time=

Average rate of change in height=
=20ft/sec
Hence Required Rate of change is -20 ft/sec, because its falling(In the direction of negative y-aixs)