11 Find the eigenvalues of the matrix $$\mathbf { A } = \left( \begin{array} { r r r } - 1 & 1 & 4
1 & 1 & - 1
2 & 1 & 1 \end{array} \right)$$ and corresponding eigenvectors. The matrix \(\mathbf { B }\) is defined by $$\mathbf { B } = \mathbf { A } - k \mathbf { I } ,$$ where \(\mathbf { I }\) is the \(3 \times 3\) identity matrix and \(k\) is a real number. Find a non-singular matrix \(\mathbf { P }\) and a diagonal matrix \(\mathbf { D }\) such that $$\mathbf { B } ^ { 3 } = \mathbf { P D } \mathbf { P } ^ { - 1 } .$$