Emerson and his friends love rowing and don't miss the sport on weekends. Starting in the morning, Emerson rowed and was 6 miles away from his starting point on a particular weekend. He continued for another 15 miles at a constant speed, only stopping for a while for rest before covering the remaining 18 miles. What's the total distance covered by Emerson on his trip?
Please reason step by step, and put your final answer within \boxed{}.
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