Let a line \( L \) be perpendicular to both the lines \( L_{1} : \, \dfrac{x+1}{3} = \dfrac{y+3}{5} = \dfrac{z+5}{7} \) and \( L_{2} : \, \dfrac{x-2}{1} = \dfrac{y-4}{4} = \dfrac{z-6}{7} \). If \( \theta \) is the acute angle between the lines \( L \) and \( L_{3} : \, \dfrac{x-8/7}{2} = \dfrac{y-4/7}{1} = \dfrac{z}{2} \), then \( \theta \) is equal to:
