To find the area of a circle, you can use the formula:
\[ \text{Area} = \pi r^2 \]
where \( r \) is the radius of the circle. The radius is half of the diameter.
Given that the diameter is 21 mi, the radius \( r \) is:
\[ r = \frac{21}{2} = 10.5 , \text{mi} \]
Now, substituting the radius into the area formula with \( \pi \) approximated as 3.14:
\[ \text{Area} = 3.14 \times (10.5)^2 \]
Calculating \( (10.5)^2 \):
\[ (10.5)^2 = 110.25 \]
Now plug this back into the area formula:
\[ \text{Area} = 3.14 \times 110.25 \]
Calculating this gives:
\[ \text{Area} \approx 346.19 , \text{mi}^2 \]
So the correct response is:
346.19 mi.²