Why do satellites with greater altitude travel at a slower speed? For example, a geostationary satellite travels slower than the ISS. What is going on?
Kepler's third law in this situation tells us that
$$ \frac{R^3}{P^2} = \frac{GM}{4\pi^2}, $$
where $R$ is the orbital radius (or semi-major axis for an elliptical orbit), $P$ is the orbital period and $M$ is the mass of the Earth (actually, it is the mas of the Earth plus the mass of the satellite, but we can neglect the latter). Thus the orbital period $P \propto R^{3/2}$.

To calculate the orbital speed, we divide the orbital circumference by the orbital period
$$ v = \frac{2\pi R}{P}. $$

Then substituting for $P$ from Kepler's third law, we get
$$ v = \sqrt{\frac{GM}{R}}$$

Thus, the larger the orbital radius, the slower the orbital speed.

Or you can just use Newton's second law and equate the centripetal acceleration to the gravitational force:
$$ m \frac{v^2}{R} = G\frac{Mm}{R^2},$$
where $m$ is the mass of the satellite, which leads directly to the same result.