Answer
$$\theta=43.89^o.$$
Work Step by Step
Since $ n_1=\langle 1,2,1\rangle $ and $ n_2=\langle 4,1,3\rangle $, then we have
\begin{align*}
\cos \theta &=\frac{n_1 \cdot n_2}{\|n_1\|\|n_2\|}\\
&=\frac{4+2+3}{\sqrt{1+4+1}\sqrt{16+1+9}}\\
&=\frac{9}{\sqrt{6}\sqrt{26}}.
\end{align*}
That is $$\theta=43.89^o.$$
