a
$\left| {\begin{array}{*{20}{c}}
1&1&{ - 1}\\
{m - 1}&{{m^2} - 7}&{ - 5}\\
{m - 2}&{2m - 5}&0
\end{array} = 0} \right.$
$m^{3}-4 m^{2}+5 m-6=0$
$\left(m^{2}-m+2\right)(m-3)=0$
$\mathrm{D}<0$ of $\mathrm{m}^{2}-\mathrm{m}+2=0$
so, $m=3,$ but at $\mathrm{m}=3$ lines are parellel, so no. of values of $\mathrm{m}$ is zero.