Explanation

(b) $V = \frac{1}{4\pi\varepsilon_{0}}\left( \frac{\mathcal{Q}_{1}}{r_{1}} + \frac{\mathcal{Q}_{2}}{r_{2}} + \frac{\mathcal{Q}_{3}}{r_{3}} \right)$
$= \frac{1}{4\pi\varepsilon_{0}}\left( \frac{33 \times 10^{- 9}}{93 \times 10^{- 3}} - \frac{51 \times 10^{- 9}}{\sqrt{2} \times 93 \times 10^{- 3}} + \frac{47 \times 10^{- 9}}{93 \times 10^{- 3}} \right)$
$= \frac{1}{4\pi\varepsilon_{0}} \times \frac{10^{- 9}}{93 \times 10^{- 3}}\left( 33 - \frac{51}{\sqrt{2}} + 47 \right)$
$\approx 4 \times 1000\ V = 4\ kV$