Let the homogeneous linear $A X=B$ be given by
$\begin{array}{lll}\mid 1 & 2 & 0 \mid & \left|x_{1}\right| \quad|0| \\ \mid 0 & 1 & 3 \mid & \left|x_{2}\right|=|0| \\ \mid 2 & 1 & 3 \mid & \left|x_{3}\right| \quad|0|\end{array}$
Show that $A$ has only the trivial solution, $\left(x_{1}, x_{2}, x_{3}\right)=(0,0,0)$.