9875 \times 10^9 N m^2/C^2$ is Coulomb's constant, $Q_1$ and $Q_2$ are the charges, $r_{21}$ is the distance between the charges, and $\hat{r}_{21}$ is the unit vector pointing from $Q_2$ to $Q_1$.
First, find the vector $\vec{r}_{21} = \vec{r_1} - \vec{r_2} =
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