Answer
-1.0 volts.
As expected, the final potential is lower than the initial potential to stop the moving electron.
Work Step by Step
Energy is conserved. The change in electrical potential energy is the negative of the change in kinetic energy.
The change in electric potential energy is q multiplied by the change in potential.
$$\Delta PE=q \Delta V=-\Delta KE$$
The final KE is zero.
$$\Delta V=\frac{-\Delta KE}{q}=\frac{KE_i-KE_f}{q}=\frac{mv_i^2}{2q}$$
$$=\frac{(9.11\times 10^{-31}kg)(6.0\times10^5m/s)^2}{2(-1.60\times 10^{-19}C)}$$
$$=-1.0V$$