Answer
$\text{approximately }0.13\text{ }km$
Work Step by Step
Substituting $D(h)=40,$ in the given function, $
D(h)=111.7\sqrt{h}
,$ results to
\begin{array}{l}\require{cancel}
40=111.7\sqrt{h}
\\
\dfrac{40}{111.7}=\sqrt{h}
.\end{array}
Squaring both sides, the equation above is equivalent to
\begin{array}{l}\require{cancel}
\left( \dfrac{40}{111.7} \right)^2=\left( \sqrt{h} \right)^2
\\
h\approx0.13
.\end{array}
Hence, the height, $h,$ is $
\text{approximately }0.13\text{ }km
.$
