\(g_{2}=\frac{G M(R-d)}{R^{3}} \ldots(2)\)
\(g_{1}=g_{2}\)
\(\frac{G M}{\left(\frac{3 R}{2}\right)^{2}}=\frac{G M(R-d)}{R^{3}}\)
\(\Rightarrow \frac{4}{9}=\frac{( R - d )}{ R }\)
\(4 R=9 R-9 d\)
\(5 R =9 d \Rightarrow \frac{ d }{ R }=\frac{5}{9}\)