You must mean whether they span or not the space of all polynomials of degree 2 or less.
Just write each given polynomial as a vector , say , etc.
Now form a 3 x 4 (or 4 x 3) matrix with the above vectors and check you get a rank 2 matrix, thus 2 vectors out of
the four are linearly independent and thus they span a 2-dimensional space, which thus
cannot be all of P_2 since this space'sdimension is 3 .