Answer
$r=\pm\sqrt{k\frac{q_{1}q_{2}}{F}}$
Work Step by Step
We solve the equation for $r$:
$F=k\displaystyle \frac{q_{1}q_{2}}{r^{2}}$
First we multiply by $r^2$:
$F*r^2=k\displaystyle q_{1}q_{2}$
Next we divide by $F$:
$r^{2}=k\displaystyle \frac{q_{1}q_{2}}{F}$
Finally, we solve for $r$:
$r=\pm\sqrt{k\frac{q_{1}q_{2}}{F}}$
