\(T =2 \pi \sqrt{\frac{\ell}{ g }}\)
At height \(h = R\)
\(g ^{\prime}=\frac{ g }{\left(1+\frac{ h }{ R }\right)^2}=\frac{ g }{4}\)
\(xT =2 \pi \sqrt{\frac{\ell}{( g / 4)}}\)
\(\Rightarrow xT =2 \times 2 \pi \sqrt{\frac{\ell}{ g }}\)
\(\Rightarrow xT =2 T \Rightarrow x =2\)