(a) Work done $W = 3 \times 10^{- 6}\left( V_{A} - V_{B} \right);$ where
$V_{A} = 10^{10}\left\lbrack \frac{\left( - 5 \times 10^{- 6} \right)}{15 \times 10^{- 2}} + \frac{2 \times 10^{- 6}}{5 \times 10^{- 2}} \right\rbrack = \frac{1}{15} \times 10^{6}\ volt$
And $V_{B} = 10^{10}\left\lbrack \frac{\left( 2 \times 10^{- 6} \right)}{15 \times 10^{- 2}} + \frac{5 \times 10^{- 6}}{5 \times 10^{- 2}} \right\rbrack = - \frac{13}{15} \times 10^{6}\ volt$
$\therefore W = 3 \times 10^{- 6}\left\lbrack \frac{1}{15} \times 10^{6} - \left( - \frac{13}{15} \times 10^{6} \right) \right\rbrack = 2.8\ J$