The formula to find the period of orbit of a satellite around a planet is (t^2) = (4pi^2/ gm) r3 where r is the orbit’s mean radius, m is the mass of the planet, and g is the universal gravitational constant. if you are given all the values except r, how do you rewrite the formula to solve for r?

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.