The force $\vec{F}=\frac{kq_1 q_2}{r^2}\hat{r}$
So the work done from $r_0$ to $r$ is

$W=\int \vec{F}\cdot d\vec{s}=\int_{r_0}^{r} \frac{kq_1 q_2}{r'^2}dr'=-\frac{kq_1 q_2}{r}|_{r_0}^{r}=kq_1 q_2 (\frac{1}{r_0}-\frac{1}{r})$