Compute the rightmost product:
[tex]\begin{bmatrix}1&2&0\\2&0&4\\0&4&-1\end{bmatrix}\begin{bmatrix}2\\2\\k\end{bmatrix}=\begin{bmatrix}1\times2+2\times2+0\times k\\2\times2+0\times2+4\times k\\0\times 2+4\times2+(-1)\times k\end{bmatrix}=\begin{bmatrix}6\\4k+4\\8-k\end{bmatrix}[/tex]
Then
[tex]\begin{bmatrix}2&2&k\end{bmatrix}\begin{bmatrix}6\\4k+4\\8-k\end{bmatrix}=2\times6+2\times(4k+4)+k\times(8-k)=-k^2+16k+20[/tex]
Use whatever methods you like to solve the remaining quadratic equation:
-k ² + 16k + 20 = 0 → k = 8 ± 2√21
