(a)
The required plane is $\left| \begin{matrix}
x - 3 & y - 6 & \mathcal{z} - 4 \\
3 - 3 & 2 - 6 & 0 - 4 \\
1 & 5 & 4
\end{matrix} \right| = 0$
$\Rightarrow \ \left| \begin{matrix}
x - 3 & y - z - 2 & \mathcal{z} - 4 \\
0 & 0 & - 4 \\
1 & 1 & 4
\end{matrix} \right| = 0$ (Operating $C_{2} \rightarrow C_{2} - C_{3}$)
$\Rightarrow 4\left( x - 3 - y + \mathcal{z} + 2 \right) = 0$
$\Rightarrow x - y + \mathcal{z} = 1$