Consider a general 2nd-degree polynomial . We must show that it can be written as a linear combination of the vectors in S:
So you can conclude now?
All you need for this question is the fact that V has dimension 3, and so cannot be spanned by a set with only 2 elements. I think the question has confused people because you have used P2 for both one of the elements of S and for the set of all polynomials with degree less than or equal to 2