Answer
$300$ $eV$
Work Step by Step
If an electron moves though a region with a electric potential $V$, the potential energy $(U)$ of the electron is given by
$U=-eV$, in which $e$ is the magnitude of an electron's charge.
Here, $V=-200$ $V$, $\therefore U=(1.6\times 10^{-19})\times200$ $J$$=200$ $eV$
Total energy of the electron is given by
$E=500$ $eV$
Therefore, the kinetic energy of the electron
$K=E-U$
or, $K=(500-200)$ $eV$
or, $K=300$ $eV$
