$-24$

$\left(\begin{matrix}-1&3&7\\2&-3&0\\-2&3&8\end{matrix}\right)$

$\left(\begin{matrix}1&\frac{1}{8}&-\frac{7}{8}\\\frac{2}{3}&-\frac{1}{4}&-\frac{7}{12}\\0&\frac{1}{8}&\frac{1}{8}\end{matrix}\right)$

$4$

$\left(\begin{matrix}-1&2&-2\\3&-3&3\\7&0&8\end{matrix}\right)$

$3,3$

$\left(\begin{matrix}1&0&0\\0&1&0\\0&0&1\end{matrix}\right)$

$\left[\begin{array}{rrr}-24 & -3 & 21 \\ -16 & 6 & 14 \\ 0 & -3 & -3\end{array}\right]$