The next number could be anything. It depends what the "rule" for the sequence is. Until we are given the rule, we cannot be assured of getting the "expected" answer.
What sorts of sequences have you worked on in class recently? What sorts of techniques have you learned?
For instance, with these three data points, it would be no trouble to find a quadratic equation that would work -- for these three points. There is, of course, absolutely no guarantee that our next number would be what the exercise's author had in mind.
Please reply with your work and reasoning. Thank you.
Eliz.