Three Dimensional Geometry

$$ \begin{aligned} &\text { Consider two skew lines in 3D space. }\\ &M_1: \frac{x-1}{1}=\frac{2-y}{1}=\frac{z-5}{1} \text { and } M_2: \frac{x+3}{1}=\frac{y-7}{2}=\frac{z+4}{1} \end{aligned} $$

Let $L_1$ be the line of shortest distance (common perpendicular) between $M_1$ and $M_2$

If $L_2$ is a line parallel to the vector $\vec{b}=\hat{\jmath}+\hat{k}$,

Then the acute angle $\boldsymbol{\theta}$ between the lines $L_1$ and $L_2$ is: