A negative rate of change means the s value is decreasing as t increases, a negative slope or ds/dt < 0. Yes, thats what they want to know?
s(t)=-4.9t^2+40
"Find the average rate of change of s over the time interval [1,2]. What does this represent?"
The average rate of change, I already found. It's -14.7. But if it's negative, what does that mean exactly? & Is that what the question means when it asks "What does this represent?"