The relevant equation to use in solving this problem, from Newton's law of gravitation, is Force = ( universal constant of gravitation x mass of earth x mass of moon)/ (distance between earth and moon squared), or F = Gm_{Earth}m_{Moon}/r ^{2}.The first step would be to ensure all units for the data provided are in SI units of metres (distances) and kilograms (mass). The distance is stated in km, so this will need to be changed to 3.844 x10^{8} m. The masses are already in kg so are fine to use as given. Then, the numbers get put into the equation as follows; F = (6.67 x10^{-11} x 6 x10^{24} x 7.3 x10^{22})/(3.844 x10^{8})^{2}= 1.98 x 10^{20} Newtons, N. (making sure to use the correct unit for force, which comes about when all SI units are used in equation.