#### Answer

$a.\qquad\left\{\begin{aligned}
x+y-3z&=8\\
y-3z&=5\\
z&=-1\end{aligned}\right.$
$b.\qquad (3,2,-1)$

#### Work Step by Step

$a.\left\{\begin{aligned}
x+y-3z&=8\\
y-3z&=5\\
z&=-1\end{aligned}\right.$
$b.$
The third equation gives $z=-1$.
Back-substituting $z=-1$ into the second equation,
$y-3(-1)=5\quad \Rightarrow\quad y=2.$
Back-substituting $z=-1 $and $y=2$ into the first equation,
$x+2-3(-1)=8\Rightarrow x=3.$
The solution is $(3,2,-1)$