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 \( -\alpha \left[ \dfrac{ql}{r^{2}} \right] \times 10^{9} \, \text{V} \), where the value of \( \alpha \) is ________. [Use \( \dfrac{1}{4\pi\varepsilon_{0}} = 9 \times 10^{9} \, \text{N m}^{2} \text{C}^{-2} \)]
