The distance between charges $+q$ and $-q$ is $2 l$ and between $+2 q$ and $-2 q$ is $4 l$. The electrostatic potential at point $P$ at a distance $r$ from center $O$ is $$-\alpha\left[\frac{q l}{r^2}\right] \times 10^9 \mathrm{~V}$$, where the value of $\alpha$ is $\underline{\hspace{2cm}}$. (Use $$\frac{1}{4 \pi \varepsilon_0}=9 \times 10^9 \mathrm{~Nm}^2 \mathrm{C}^{-2}$$)