Sam says that the membership in the science club is growing linearly. at the first time meeting there were 12 members. by the fifth meeting, the club had grown to 18 members. assuming this growth continues, how many member can they expect at the seventh meeting?

At the seventh meeting there would be 20 members. If you think about the question carefully there were 12 members at the first meeting, then they skipped 6 meetings, with each meeting being skipped or not written down they added 1 member, so you would add 2 members for the next 2 meetings. making 20 members. 
Hope this helps :)

The correct answer is: 2