Work done by external agent : $W_{ext} = \Delta U;$ $\Delta U \rightarrow$ Change in potential energy in taking the charge from initial to final configuration $\Rightarrow W_{ext} = \frac{1}{4\pi\epsilon_0} \frac{q_1q_2}{r_f} - \frac{1}{4\pi\epsilon_0} \frac{q_1q_2}{r_i}$ Now, $r_f = \sqrt{(2-0)^2 + (2-0)^2 + (1-0)^2} = 3 \text{ m}$ $r_i = \sqrt{(4-0)^2 + (4-0)^2 + (2-0)^2} = 6 \text{ m}$ $\therefore W_{\text{ext}} = (9 \times 10^9) \times (10^{-8} \times 2 \times 10^{-6}) \left[ \frac{1}{3} - \frac{1}{6} \right]$ $= 3 \times 10^{-5}$ $= 30 \times 10^{-6} \text{J}$