$M =$ mass of earth

$M _{1}= mass$ of shaded portion

$R =$ Radius of earth

$M_{1}=\frac{M}{\frac{4}{3} \pi R^{3}} \cdot \frac{4}{3} \pi(R-h)^{3}$

$=\frac{M(R-h)^{3}}{R}$

Weight of body is same at $P$ and $Q$

i.e. $mg _{ P }= mg _{ Q }$

$g _{ P }= g _{ Q }$

$\frac{G M_{1}}{(R-h)^{2}}=\frac{G M}{(R+h)^{2}}$

$\frac{G M(R-h)^{3}}{(R-h)^{2} R^{3}}=\frac{G M}{(R+h)^{2}}$

$( R – h )( R + h )^{2}= R ^{3}$

$R^{3}-h R^{2}-h^{2} R-h^{3}+2 R^{2} h-2 R h^{2}=R^{3}$

$R^{2}-R h^{2}-h^{3}=0$

$R^{2}-R h-h^{2}=0$

$h ^{2}+ Rh – R ^{2}=0 \Rightarrow h =\frac{- R \pm \sqrt{ R ^{2}+4 R ^{2}}}{2}$

ie $h =\frac{- R +\sqrt{5} R }{2}=\left(\frac{\sqrt{5}-1}{2}\right) R$