Answer
The work done by an external agent is $2.3\times 10^{-13}~J$
Work Step by Step
We can find the electric potential energy:
$U = \frac{k~q_1~q_2}{r}$
$U = \frac{(9.0\times 10^9~N~m^2/C^2)(1.6\times 10^{-19}~C)(1.6\times 10^{-19}~C)}{1.0\times 10^{-15}~m}$
$U = 2.3\times 10^{-13}~J$
When the charges are at an infinite separation, the electric potential energy is zero. Therefore, the work done by an external agent is $2.3\times 10^{-13}~J$
