A linear transformation from a vector space of polynomials

Let V denote the real vector space of polynomials in x of degree at most 3. Let B={1,x,x2,x3} be a basis for V and T:VV be the function defined by T(f(x))=f(x)+f(x).

  1. Prove that T is a linear transformation.
  2. Find [T]B, the matrix representation for T in terms of the basis B.
  3. Is T diagonalizable? If yes, find a matrix A so that A[T]BA1 is diagonal, otherwise explain why T is not diagonalizable.