Explanation
Active Voice: They will cast the votes.
Passive Voice: Votes will be cast by them.
Future Indefinite Tense-Passive Voice
Convert the object of the Active Voice Sentence into the subject of the Passive Voice sentence.
Then use helping verb will be/shall be according to the subject of the Passive Voice Sentence.
Then use the 3rd form of Verb.
After that, use the word “by”, in order to show the doer.