Mark and his sister Chris both leave their house for school at the same time. Mark travels at the same speed as Chris, who walks 3 miles per hour. After walking 3 miles Mark has to turn around and go home because he forgot his lunch. If the distance from their house to the school is 9 miles, how much longer does Mark spend walking than Chris? Let's answer step by step:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
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First find the total number of miles Mark walks to get his lunch by multiplying the distance he walks back to the house by 2: 3 miles * 2 = 6 miles. Then find the total distance Mark walks by adding that distance to the distance between the house and the school: 6 miles + 9 miles = 15 miles. Now divide the total distance Mark walks by his speed to find how many hours he spends walking: 15 miles / 3 miles / hour = 5 hours. Now divide the total distance Chris walks by her speed to find the total time she spends walking: 9 miles / 3 miles / hour = 3 hours. Now subtract the time Chris spends walking from the time Mark spends walking to find the difference: 5 hours - 3 hours = 2. The answer: 2.