A vector space of square matrices

Let Mn(R) be the vector space of all n×n matrices with real entries. We say that A,BMn(R) commute if AB=BA.

  1. Fix AMn(R). Prove that the set of all matrices in Mn(R) that commute with A is a subspace of Mn(R).
  2. Let A=[1111]M2(R) and let WM2(R) be the subspace of all matrices of M2(R) that commute with A. Find a basis of W.