given is of the form $Ax=b$
$Ax_1=b_1$,$Ax_2=b_2$,
$Ax_3=?$
\[
x_3= \begin{pmatrix} 6 \\ 7 \\ 8 \end{pmatrix}=-1\begin{pmatrix} 0 \\ 1 \\ 2 \end{pmatrix} + 2 \begin{pmatrix} 3 \\ 4 \\ 5 \end{pmatrix}
\]
That is question asking $A(-x_1+2x_2)=?$
$A(-x_1+2x_2)=-b_1+2b_2$
\[
-1\begin{pmatrix} 1 \\ 0 \\ 0 \end{pmatrix} + 2 \begin{pmatrix} 0 \\ 1 \\ 0 \end{pmatrix} = \begin{pmatrix} -1 \\ 2 \\ 0 \end{pmatrix}
\]
