Well, you're not told to find an orthonormal basis, or I'd say use the Gram-Schmidt procedure. You're just told to find a basis. What I would do is take the vectors you're given in part a), for example, set an arbitrary linear combination of them equal to zero, and solve for the coefficients. There may be more to it than that, but that's where I'd start. Your goal is to eliminate any superfluous vectors. It may be that the space spanned by the vectors in part a) is only 2-dimensional. It's certainly at least 1-dimensional!