(b)
Coordinates of $L$ and $M$ are $(0,\ b,\ c)$ and $(a,\ 0,\ c)$, respectively. Therefore, the equation of the plane passing through (0, 0, 0), $(0,\ b,\ c)$ and $(a,\ 0,\ c)$ is
$\left| \begin{matrix}
x - 0 & y - 0 & \mathcal{z} - 0 \\
0 & b & c \\
a & 0 & c
\end{matrix} \right| = 0\ or\ \ \frac{x}{a} + \frac{y}{b} - \frac{\mathcal{z}}{c} = 0$