What I would do is convert the fractions to decimals (just personal preference) making the trombone 8.93 feet and the french horn 17.21 feet. The tuba would be 0.79 feet longer than the french horn, and the french horn would be 8.23 feet longer than the trombone. However, if you need the answer to remain a fraction, the tuba would be 11/14 feet longer than a french horn, and a french horn would be 8 3/14 feet longer than a trombone.