Answer
$k=24.$
Work Step by Step
A matrix is singular if and only if its determinant is $0$.
We know that for a matrix
$
\left[\begin{array}{rrr}
a & b & c \\
d &e & f \\
g &h & i \\
\end{array} \right]
$
the determinant, $D=a(ei-fh)-b(di-fg)+c(dh-eg).$
Hence here $D=1(-1\cdot k-0\cdot2)-0(2\cdot k-0\cdot4)+3(2\cdot2-(-1)\cdot4)=1(-k)-0(2k)+3(8)=0\\-k+24=0\\k=24.$
