9. 在平行六面体ABCD−A′B′C′D′ABCD-A\prime B\prime C\prime D\primeABCD−A′B′C′D′中,AB=4AB=4AB=4,AD=3AD=3AD=3,AA′=5AA\prime =5AA′=5,∠BAD=90∘\angle BAD=90\circ∠BAD=90∘,∠BAA′=∠DAA′=60∘\angle BAA\prime =\angle DAA\prime =60\circ∠BAA′=∠DAA′=60∘,则AC′AC\primeAC′的长((( )))