\( \Large  17^{3.5} \times 17^{7.3} \div 17^{4.2} = 17^{?} \)

=> \(  \Large 17^{3.5 + 7.3 - 4.2} = 17^{?} \)

=> \(  \Large 17^{6.6} = 17^{?} \)

Therefore ? = 6.6