$-12$

$\left(\begin{matrix}5&1&7\\1&2&0\\5&-2&8\end{matrix}\right)$

$\left(\begin{matrix}-\frac{4}{3}&\frac{11}{6}&\frac{7}{6}\\\frac{2}{3}&-\frac{5}{12}&-\frac{7}{12}\\1&-\frac{5}{4}&-\frac{3}{4}\end{matrix}\right)$

$15$

$\left(\begin{matrix}5&1&5\\1&2&-2\\7&0&8\end{matrix}\right)$

$3,3$

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

$\left[\begin{array}{rrr}16 & -22 & -14 \\ -8 & 5 & 7 \\ -12 & 15 & 9\end{array}\right]$