Answer
$r = \pm \sqrt{\dfrac{GmM}{F}}$
Work Step by Step
Multiply $r^2$ on both sides of the equation to obtain:
$Fr^2=GmM$
Divide by $F$ on both sides of the equation to obtain:
$r^2=\dfrac{GmM}{F}$
Take the square root of both sides to obtain:
$r = \pm \sqrt{\dfrac{GmM}{F}}$
