remark
Somewhere you need to find the area of the pentagon. It's the base. I'm not sure what the height is (8 or 8.3) 8.3 is for certain, but then I don't know what the 8 is.
Formulas
V = Bh
Area of B = (1/4) sqrt(5(5 + 2sqrt(5) ) * a^2
Sub and solve
Find the base first
a = 12 miles
Area of B = 0.25 * sqrt(25 + 10sqrt(5) ) * 12^2
B = 0.25 sqrt(25 + 10*3.1623) * 144
B = 0.25 sqrt(25 + 31.623) * 144
B = 0.25 sqrt( 56.623) * 144
B = 0.25 * 7.5248 * 144
B = 270.893
Find the Volume
V = B * h
V = 270.893 * 8.3
V = 2248.4 <<<<<<<<< Answer
I will put an alternate answer in the comment section