The distance between charges $+q$ and $-q$ is $2l$ and between $+2q$ and $-2q$ is $4l$. The electrostatic potential at point $P$ at a distance $r$ from centre $O$ is $-\alpha \dfrac{ql}{r^2} \times 10^9$ V, where the value of $\alpha$ is $\_\_\_\_$. (Use $\dfrac{1}{4\pi\varepsilon_0} = 9 \times 10^9$ N$^2$ m$^2$)