Answer:
[tex]\large\boxed{ r=\sqrt{\dfrac{GM_1M_2}{F_g}}}[/tex]
Step-by-step explanation:
[tex]F_g=\dfrac{GM_1M_2}{r^2}\qquad\text{multiply both sides by}\ r^2\\\\F_gr^2=r^2\!\!\!\!\!\!\diagup\cdot\dfrac{GM_1M_2}{r^2\!\!\!\!\!\!\diagup}\\\\F_gr^2=GM_1M_2\qquad\text{divide both sides by}\ F_g\\\\\dfrac{F_g\!\!\!\!\!\!\diagup r^2}{F_g\!\!\!\!\!\!\diagup}=\dfrac{GM_1M_2}{F_g}\\\\r^2=\dfrac{GM_1M_2}{F_g}\Rightarrow r=\sqrt{\dfrac{GM_1M_2}{F_g}}[/tex]
