Gravitational field intensity will be zero,
\(\Rightarrow \frac{G m}{r^2}=\frac{2 G m}{(d-r)^2}\)
\(\Rightarrow \frac{1}{r}=\frac{\sqrt{2}}{d-r}\)
\(\Rightarrow d-r=\sqrt{2} r\)
\(\Rightarrow r(1+\sqrt{2})=d\)
\(\Rightarrow r=\frac{d}{(1+\sqrt{2})}\)