(b) The value of $g$ at the height $h$ from the surface of earth
$g^{'} = g\left( 1 - \frac{2h}{R} \right)$
The value of $g$ at depth $x$ below the surface of earth
$g' = g\left( 1 - \frac{x}{R} \right)$
These two are given equal, hence $\left( 1 - \frac{2h}{R} \right) = \left( 1 - \frac{x}{R} \right)$
On solving, we get $x = 2h$