Answer
$$\theta=54.73^o.$$
Work Step by Step
By making use of the result from problem 36 (d), we get $ n_1=\langle 1,1,1\rangle $ and we have $ n_2=\langle 1,0,0\rangle $; then we get
\begin{align*}
\cos \theta &=\frac{n_1 \cdot n_2}{\|n_1\|\|n_2\|}\\
&=\frac{1+0+0}{\sqrt{1+1+1}\sqrt{1+0+0}}\\
&=\frac{1}{\sqrt{3} }.
\end{align*}
That is $$\theta=54.73^o.$$
