So we want to know what is the sum of 5 feet 9 inches and 12 feet 3 inches. To know that we need to know that there are 12 inches in one foot. Now we add feet with feet and inches with inches: 5 + 12 feet 9 inches + 3 inches = 17 feet 12 inches = 18 feet 0 inches or just 18 feet.
