Area of a circle = pi*r^2.
Here, A = 380.13 mi. Unfortunately, "mi" is NOT a unit of area.
So I will assume that the problem should read A = 380.13 sq. miles.
380.12 mi^2
Thus, 380.13 mi^2 = 3.14*r^2, and r^2 = ------------------- 121.057 mi^2
3.14
The radius is the sqrt of 121.057 mi^2: 11.00 miles.
The diam. is twice that, or d = 22.00 miles.