b
$\Delta=\left|\begin{array}{ccc}1 & 4 & -1 \\ 7 & 9 & \mu \\ 5 & 1 & 2\end{array}\right|=0$
$\Rightarrow(18-\mu)-4(14-5 \mu)-(7-45)=0 \Rightarrow \mu=0$
$\Delta=\Delta_{\mathrm{x}}=\Delta_{\mathrm{y}}=\Delta_{\mathrm{z}}=0$ (For infinite solution)
$\Delta_x=\left|\begin{array}{ccc}\lambda & 4 & -1 \\ -3 & 9 & \mu \\ -1 & 1 & 2\end{array}\right|=0$
$ \lambda(18-\mu)-4(-6+\mu)-1(-3+9)=0 $
$ 18 \lambda+24-6=0 \Rightarrow \lambda=-1$