$\left| {\begin{array}{*{20}{c}}
1&{ - 2}&k\\
2&1&1\\
3&{ - 1}&{ - k}
\end{array}} \right| - 0$
$ \Rightarrow k = \frac{{ - 1}}{2}$
Also consider
$x - 2y + k = 1$ and $2x + y + z = 2$
$ \Rightarrow 2x - 4y - z - 2$
$2x + y + z = 2$
$ \Rightarrow 4x - 3y = 4$