average rate of change is the slope (of the line) between the two coordinates
f(t) = -16t² + 48t + 100
f(3) = -16(3)² + 48(3) + 100 = -144 + 144 + 100 = 100 (3, 100)
f(5) = -16(5)² + 48(5) + 100 = -400 + 240 + 100 = -60 (5, -60)
m = =
=
= -80
Answer: -80 ft/sec