Final answer:
The expression a - 21 represents the number of children's tickets sold, while 'a' represents the total number of adult tickets sold.
Step-by-step explanation:
The expression a - 21 represents the number of children's tickets sold. In this case, 'a' represents the total number of adult tickets sold. Since the student sold 21 more adult tickets than children's tickets, the expression a - 21 gives us the number of children's tickets. To find the value of 'a', we need to add 21 to the number of children's tickets.