The ScientificConstants package contains values of physical constants. A  list of constants can be found here.

Use GetConstant to get the definition of the gravitational constant.

To just return the numerical value,

The expression for circular orbital velocity around a spherically symmetric body is:

where, G is the gravitational constant, M is the mass of the body, and R is the radius of orbit.

Now, for geostationary orbit, an orbital velocity of 2 π R meters per day is required, which in meters per second is:

Equate this to $v_{\mathrm{CircOrb}}$.

Solve for R. To disregard the non-real solutions, use the RealDomain package.

Replace G with its value and M with the mass of the Earth, then evaluate.