If you have three unknowns, then it means you need three pieces of independent information. Since your polynomial is y(x) = ax^2 + bx + c, it means you need three pieces of information to get your a, b, and c.
You have been given two pieces but you need another. Is there something else that you are assuming? Is the quadratic symmetric around the point x = 0? (If it is then that is your third piece of information)