Force(centripetal)=Force(gravity)
(mv^2)/r=(GMm)/r^2
because v=(2pi r)/T
substituting v and re-arranging:
r^3=(GMT^2)/(4pi^2)
r^3=(6.67*10^-11*6*10^24*(24*60*60)^2) / (4pi^2)
r=4.23*10^7m
from earth surface= 4.23*10^7 - 6.37*10^6
=3.59*10^7m above earth's surface