Explanation

(a) Rest energy of an electron $= m_{e}c^{2}$
Here $m_{e} = 9.1 \times 10^{- 31}kg$ and $c =$ velocity of light
$\therefore$ Rest energy $= 9.1 \times 10^{- 31} \times \left( 3 \times 10^{8} \right)^{2}joule$
$= \frac{9.1 \times 10^{- 31} \times \left( 3 \times 10^{8} \right)^{2}}{1.6 \times 10^{- 19}}eV = 510\ keV$