If the expression is completed, there are several options for the answer:

Consider the number of students in the class who are not booked.

If there are only three people, neither of them has an appointment.

If there are less than three people, this question is illogical.

If more than 3 people are less than or equal to 2 1 and recorded as one person, both of them will be booked: 18+23-(44-a).

If it is more than 2 1, this question is illogical.