However I suspect you are learning about arithmatic and geometric series, which have the following formulas (respectively)
Each of these equations has two unknowns in it ( and respectively) so we need two numbers in the series in order to solve for them. BUT we can always write both an arithmatic and geometric series between any two numbers, so to distinguish between the cases we need an extra number. So your answer is likely that you need three numbers from the series.
However, not all series are so nice. For example, the Fibbonacci series
fits neither of these types of series, and of course the series of numbers 3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5, ... representing the digits of follow no known pattern. So your problem is insolvable in general.