See the attached graphic to see this solved for "r". You should be very careful about stating the units for each of those variables. time in seconds radius in meters mass in kilograms G = 6.674 x 10^-11 m^3 / (kilograms * seconds²) Also, it would be worth your while to go to this web page http://www.1728.org/kepler3a.htm It has a calculator and other information about Kepler's Third Law.