a) $$\begin{aligned} \mathrm{v}= & \frac{2 \pi r}{t}=\frac{2 \pi \times 4,2 \times 10^{4}}{24}=1,05 \times 10^{4} \mathrm{~km} / \mathrm{h} \\ & a_{\mathrm{c}}=\frac{\mathrm{v}^{2}}{r}=\left(1,05 \times 10^{4}\right)^{2} \frac{1}{4,2 \times 10^{4}}=2625 \mathrm{~km} / \mathrm{h}^{2} \end{aligned}$$ b) $$\begin{aligned} & \Delta E=E_{\text {geo }}-E_{\text {comum }}=-\frac{G M m}{2 r_{\text {geo }}}-\left(-\frac{G M m}{2 r_{\text {comum }}}\right)=\frac{G M m}{2}\left(\frac{1}{r_{\text {comum }}}-\frac{1}{r_{\text {geo }}}\right)= \\ & =\frac{1}{2}\left(6,7 \times 10^{-11} \frac{\mathrm{Nm}^{2}}{\mathrm{~kg}^{2}}\right)\left(6 \times 10^{24} \mathrm{~kg}\right)(200 \mathrm{~kg})\left(\frac{1}{7 \times 10^{6} \mathrm{~m}}-\frac{1}{42 \times 10^{6} \mathrm{~m}}\right)=4,8 \times 10^{9} \mathrm{~J} \end{aligned}$$