The average speed is 80 m / 17 s = 4.7 m/s. Since the final speed 5.6 m/s > 4.7 m/s, the truck could not have been slowing down; it must have been accelerating. To find the initial speed v0, use the fact that 4.7 is the average of v0 and 5.6, i.e., 4.7 = (v0 + 5.6) / 2.