To solve this, we also need the other information from your other posting of this question.
Up to Algebra 2: 38 Both:9 AP Calculus:?
The students are divided into three groups: (a) those who entered "up to algebra 2", (b) those who entered AP calculus, and (c) those who entered both. To find group (b), we must find the number of students who are not in groups (a) or (b). We can use an equation in the following form:
("algebra 2" group) + ("both" group) + ("AP" group) = (total students)
Substituting the values given, and using a for the AP group, we have
38 + 9 + a = 68
47 + a = 68
a = 68-47 = 21