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 $$-\alpha\frac{ql}{r^2} \times 10^9$$ V, where the value of $$\alpha$$ is. (Use $$\frac{1}{4\pi\varepsilon_0} = 9 \times 10^9$$ N m$$^2$$ C$$^{-2}$$)