b
Let $A=\left(\begin{array}{ccc}0 & -1 & 0 \\ 0 & 1 & -1 \\ 0 & 0 & 1\end{array}\right)=1+C$
Where $I=\left(\begin{array}{lll}1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1\end{array}\right), C=\left(\begin{array}{ccc}0 & -1 & 0 \\ 0 & 0 & -1 \\ 0 & 0 & 0\end{array}\right)$
$C^{2}=\left(\begin{array}{lll}0 & 0 & 1 \\ 0 & 0 & 0 \\ 0 & 0 & 0\end{array}\right)$
$C^{3}=\left(\begin{array}{lll}0 & 0 & 0 \\ 0 & 0 & 0 \\ 0 & 0 & 0\end{array}\right), C^{4}=C^{5}=\ldots .$
$B=7 A^{20}-20 A^{7}+2 I$
$=7(1+c)^{20}-20(1+C)^{7}+2 I$
So
$\mathrm{B} 13=7 \times{ }^{20} \mathrm{C}_{2}-20 \times{ }^{7} \mathrm{C}_{2}=910$