Four point charges -Q,-q, 2 q and 2 Q are placed, one at each...

Let $a$ be the side length of the square A B C D$.$$\begin{aligned} & A C=B D=\sqrt{a^2+a^2}=a \sqrt{2} \\ & O A=O B=O C=O D=\frac{a \sqrt{2}}{2}=\frac{a}{\sqrt{2}}\end{aligned}$Potential is a scalar quantity. Potential at the centre $O$ due to given charge configuration is\begin{aligned} & V=\frac{1}{4 \pi \varepsilon_0}\left[\frac{(-Q)}{\left(\frac{a}{\sqrt{2}}\right)}+\frac{(-q)}{\left(\frac{a}{\sqrt{2}}\right)}+\frac{(2 q)}{\left(\frac{a}{\sqrt{2}}\right)}+\frac{2 Q}{\left(\frac{a}{\sqrt{2}}\right)}\right]=0 \\ & \Rightarrow-Q-q+2 q+2 Q=0 \\ & \text{ or } Q+q=0 \text{ or } Q=-q\end{aligned}$$