Ответ:
1) They were to discuss this plan at the last meeting.
(-) They were not to discuss this plan at the last meeting.
(?) Were they to discuss this plan at the last meeting?
2) The students will be allowed to use this method.
(-) The students will not be allowed to use this method.
(?) Will the students be allowed to use this method?
3) He has to go to the office.
(-) He doesn’t have to go to the office.
(?) Does he have to go to the office?