Answer
$h=2.29$ hours
Work Step by Step
$D=5e^{-0.4h}$, hence if $D=2$, then $2=5e^{-0.4h}$.
Solve the equation above to obtain:
\begin{align*}\frac{2}{5}&=e^{-0.4h}\\
\ln{\frac{2}{5}}&=-0.4h\\
\dfrac{\ln{\frac{2}{5}}}{-0.4}&=h\\
2.29&\approx h\end{align*}.
