If the two lines \( l_1 : \dfrac{x-2}{3} = \dfrac{y+1}{-2}, \; z = 2 \) and \( l_2 : \dfrac{x-1}{1} = \dfrac{2y+3}{\alpha} = \dfrac{z+5}{2} \) are perpendicular, then an angle between the lines \( l_2 \) and \( l_3 : \dfrac{1-x}{3} = \dfrac{2y-1}{-4} = \dfrac{z}{4} \) is:
