Hostes adventu nostri territi sunt. the enemies were terrified by the arrival of our army. exercitui exercituum exercitus exercitibus

I believe the correct answer is exercitui.
You need the perfect passive form here, which is formed when you combine the verb esse in the present tense (here, sunt because of third person plural), and the appropriate participle in the same person, which in this case is exercitui - third person plural. 

Im doing good lol. you might want to pay attention to your teacher instead of your phone