Answer
$4.608\times 10^{-13}J$
Work Step by Step
We know that
$U_f=U_{12}+U_{13}+U_{23}$
$\implies U_f=\frac{kq^2}{r_{12}}+\frac{kq^2}{r_{13}}+\frac{kq^2}{r_{23}}$
As $r_{12}=r_{13}=r_{23}=r=1.5\times 10^{-15}$
$\implies U_f=\frac{3Kq^2}{r}$
We plug in the known values to obtain:
$U_f=\frac{3(9\times 10^9)(1.6\times 10^{-19})^2}{1.5\times 10^{-15}}$
$U_f=4.608\times 10^{-13}J$
$Work \space done=U_f-U_i$
$\implies Work \space done=4.608\times 10^{-13}-0=4.608\times 10^{-13}J$
