Final answer:
Joe is walking at a speed of 2.5 kilometers per hour.
Step-by-step explanation:
In order to find Joe's walking speed, we need to first determine the total distance that Jill and Joe will cover before they meet. Since Jill walks at 2 kilometers per hour and they meet after 3 hours, Jill would have covered a distance of 2 km/hr * 3 hr = 6 kilometers. Since Joe is 13.5 kilometers away from Jill, this means that Joe would have covered a distance of 13.5 km - 6 km = 7.5 kilometers before they meet.
Therefore, Joe's walking speed is the distance covered divided by the time taken, which is 7.5 km / 3 hr = 2.5 kilometers per hour.