SI. 2 Three solutions of the equation $p x+q y+b z=1$ are $(0,3 a, 1),(9 a,-1,2)$ and $(0,3 a, 0)$. Find the value of the coefficient $b$.
$$
\left\{\begin{array}{r}
3 a q+b=1 \\
9 a p-q+2 b=1 \\
3 a q=1
\end{array}\right.
$$

Sub. (3) into (1): $1+b=1$
$$
\Rightarrow b=0
$$