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.
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.