4. Let two 3 x 3 matrices A and B be given by O 4 -2 -1 A = 4 and B = -4 2 1 3 -4 -3 (a) Calculate the matrix multiplications AB and A?. (b) Knowing that a matrix is invertible if and only if its determinant is nonzero, does either A or B have an inverse? If either does, then calculate its inverse by means of the cofactor matrix