Answer
$$\theta=94.79^o.$$
Work Step by Step
Since $ n_1=\langle 1,-3,1\rangle $ and $ n_2=\langle 2,0,-3\rangle $, then we have
\begin{align*}
\cos \theta &=\frac{n_1 \cdot n_2}{\|n_1\|\|n_2\|}\\
&=\frac{2+0-3}{\sqrt{1+9+1}\sqrt{4+0+9}}\\
&=\frac{-1}{\sqrt{11}\sqrt{13}}.
\end{align*}
That is $$\theta=94.79^o.$$
