Recall: "`"pm"`" means picometre, or `10^-12` meters.

In this example,

`a = 1 ×10^-12\ "m"`

`b = 4 ×10^-12\ "m"`

`k = 9 × 10^9`

`q_1= q_2= 1.6 × 10^-19\ "C"`

So we have:

`"Work"=int_a^b (k\ q_1q_2)/(x^2) dx`

`=int_[1 times10^-12]^[4 times 10^-12] ((9 times 10^9)(-1.6 times 10^-19)^2)/(x^2) dx`

`=(2.304 times 10^-28)[-1/x]_[1 times 10^-12]^[4 times 10^-12]`

`=1.728 times 10^-16\ text[J]`