A satellite goes round the Earth once every 8 hours…

Step 1. Radius. $T = 28\,800\ \text{s}$: $r^3 = 4.014 \times 10^{14}\ \text{m}^3/\text{s}^2 \times \dfrac{(28\,800\ \text{s})^2}{4\pi^2} = 8.43 \times 10^{21}\ \text{m}^3$, so $r = 2.036 \times 10^{7}\ \text{m}$.

Step 2. Height. $20\,355\ \text{km} - 6400\ \text{km} = 13\,955\ \text{km}$.

Step 3. Speed. $v = \dfrac{2\pi \times 2.036 \times 10^{7}\ \text{m}}{28\,800\ \text{s}} = 4.44\ \text{km/s}$.