There are twice as many students in the cooking club as in the drama club. Suppose there are students in the drama club and students who are members of both clubs. Find an expression for the total number of students who are in the cooking club or the drama club but not both. Give your answer in simplest form.
When you subtract out the students in both groups, you need to subtract twice since those students were counted twice.
Indeed, there are students in the drama club and in the cooking club. But some students are in both! So, counts some students more than once. How do we correct for that?