61. Let C be defined by

$$

C = \begin{bmatrix}

2B & -B & -B \\

-B & 2B & -B \\

-B & -B & 2B

\end{bmatrix},

$$

where B =

$$

\begin{bmatrix}

-1 & -1

\end{bmatrix}

$$

;

find the characteristic roots of C.