If the two lines l_1: (x-2)/3=(y+1)/-2, z=2 and l_2: (x-1)/1=(2 y+3)/(α)=(z+5)/2 are perpendicular, then an angle between the lines l_2 and l_3:…

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