find the height above the earth where value of acceleration due to gravity is the same as that at 20m deeper inside the earth

Suppose the height is h, so the acceleration due to gravity at that height is
$g_h=\frac{GM}{{\left(r+h\right)}^2}$
and g at 20 m depth is given by
$\frac{4}{3}\mathrm{\pi \rho G}\left(r-20\right)$
So for equal we have
$$\begin{gathered} \frac{4}{3}\mathrm{\pi \rho G}\left(\mathrm{r}-20\right)=\frac{\mathrm{GM}}{{\left(\mathrm{r}+\mathrm{h}\right)}^2} \\ \left(\mathrm{r}-20\right){\left(\mathrm{r}+\mathrm{h}\right)}^2=\frac{\mathrm{M}}{\frac{4}{3}\mathrm{\pi \rho }}=\mathrm{r} \end{gathered}$$
Now putting the values of r and solving equation we get
$h=-1.2\times {10}^7 \text{and} 10$
Here negative sign does not mean any thing so the correct answer is 10 m.