Answer
$-0.27J$
Work Step by Step
We can find the required work done as follows:
$W=\Delta U=0-(\frac{Kq_1q_2}{r_{12}}+\frac{Kq_1q_3}{r_{13}}+\frac{Kq_2q_3}{r_{23}})$
$W=-K(\frac{q_1q_2}{r_{12}}+\frac{q_1q_3}{r_{13}}+\frac{q_{23}}{r_{23}})$
We plug in the known values to obtain:
$W=-(8990)[\frac{(-6.1)(2.7)}{0.25}+\frac{(-6.1)(3.3)}{0.16}+\frac{(2.7)(-3.3)}{0.30}]$
$W=-0.27J$