Two bodies of masses m and M are placed a distance d apart. The gravitational potential at the position where the gravitational field due to them is zero is V, then 

Let gravitational field be zero at a point lying at distance x from M.
$\mathrm{x}=\frac{\mathrm{d}}{\sqrt{\frac{\mathrm{m}}{\mathrm{M}}+1}};{\mathrm{V}}_{\mathrm{P}}=-\frac{\mathrm{Gm}}{\mathrm{d}-\mathrm{x}}-\frac{\mathrm{GM}}{\mathrm{x}}$
$\begin{array}{l}{\mathrm{V}}_{\mathrm{P}}=-\mathrm{G}\left[\left(\frac{\mathrm{M}\left(\sqrt{\frac{\mathrm{m}}{\mathrm{M}}}+1\right)}{\mathrm{d}}\right)+\frac{\mathrm{m}}{\mathrm{d}-\frac{\mathrm{d}}{\sqrt{\frac{\mathrm{m}}{\mathrm{M}}}+1}}\right]\\ {\mathrm{V}}_{\mathrm{P}}=-\mathrm{G}\left(\frac{\mathrm{M}\left(\sqrt{\frac{\mathrm{m}}{\mathrm{M}}}+1\right)}{\mathrm{d}}+\frac{\mathrm{m}\left(\sqrt{\frac{\mathrm{m}}{\mathrm{M}}}+1\right)}{\mathrm{d}\left(\sqrt{\frac{\mathrm{m}}{\mathrm{M}}}+1-1\right)}\right]\\ =\frac{-\mathrm{G}}{\mathrm{d}}\left(\sqrt{\mathrm{Mm}}+\mathrm{M}+\mathrm{m}\left(1+\sqrt{\frac{\mathrm{M}}{\mathrm{m}}}\right)\right)\end{array}$
$\begin{array}{l}=-\frac{\mathrm{G}}{\mathrm{d}}(\sqrt{\mathrm{Mm}}+\mathrm{M}+\mathrm{m}+\sqrt{\mathrm{Mm}})\\ \mathrm{V}=\frac{-\mathrm{G}}{\mathrm{d}}(2\sqrt{\mathrm{Mm}}+\mathrm{M}+\mathrm{m})\\ \mathrm{V}=\frac{-\mathrm{G}}{\mathrm{d}}(\sqrt{\mathrm{m}}+\sqrt{\mathrm{M}}{)}^2\end{array}$