Question $7,4\mathrm{.}1\mathrm{.}31$

Determine the following.

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}3e^{-0\mathrm{.}4x}dx$

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}3e^{-0\mathrm{.}4x}dx=($Type an exact answer.$)$