2. Let A E Rmxn be some fixed m x n matrix and let Z be the collection of n x k matrices such that for all B contained in Z, AB = 0 (where 0 is the m x k zero matrix). a) (3 pts) State what it means for the collection of matrices in Z to be a subspace of R***. b) (5 pts) Now show that Z is a subspace of R***.