The problem statement gives the correct answers for parts (a) and (b). The total number of roots of the characteristic polynomial is the dimension of the matrix: 6. The eigenvalues are the zeros of the characteristic polynomial, 3 (multiplicity 2), 6 (multiplicity 3), and -1.
(c) The matrix is not invertible when one or more eigenvalues is zero. None of yours are zero, so the matrix is invertible.