Looks like you have the right equation and all the needed information. Check to make sure you are entering everything into your calculator properly. Simply solve the equation for R.
The G in the universal law of gravitation is not the same as g=9.81m/sec which is the acceleration due to gravity at the Earth's surface. It's 6.67*10^(-11)*m^3/(kg*sec^2). Try keeping track of units.
If you say 24.713m, I'll agree with that. Put units on things. If you'd done that you would have realized 9.8m/sec^2 couldn't be the G you want. Because you wouldn't have gotten an answer in meters.