Answer
$1.62 \times 10^8 \pi \text{ or } S_{a} \approx 5.10 \times 10^8 \text{km}^2$
Work Step by Step
Earth's radius $r \approx 6.37 \times 10^6$ meters. Remember that 1 kilometer = 1000 meters. Then the radius in kilometers is
$$r \approx 6.37 \times 10^6 \text{meters} \times \cfrac{1 \text{ kilometers}}{1000 {\text{ meters}}}$$
The units cancel,
$$ \approx \frac{6.37 \times 10^6 \text{ kilometers}}{1000} = 6.37 \times 10^3 \text{ kilometers}$$
The Surface Area of a Sphere is $A_{S}= 4\pi r^2 $ then after rounding for significant figures:
$$A_{S}= 4\pi ( 6.37 \times 10^3 \text{ kilometers)}^2 = 1.62 \times 10^8 \pi$$ $$ = 5.10 \times 10^3 \text{ km}^2$$
