We can start with Newton's Law of Universal Gravitation, which is
[tex] F_{grav} = (G. m_{1} .m_{2})/d^{2}[/tex]
Otherwise known as... The force due to gravity between two objects is equal to G ( a universal constant G = 6.673 x 10-11 N m2/kg2) times the mass of the first object (kg) times the mass of the second object (kg) all divided by the distance between the two objects squared.
We can thus plug in the given values to receive the answer...
[tex]F_{grav}=(6.673x10^{-11}.(6.9)^{2})/(0.218)^2[/tex]
I converted the 21.8cm into 0.218m, for the equation calls for meters.
I hope this helps!
P.S. I'm currently enrolled in IB Physics, so if you have any more questions, feel free to contact me for help.
