Answer: 5750.17 people/square miles
to determine the population density you need to divide the population with the square miles.
A circular region with 6-mile radius would have a total area of : 3.14 x 6 miles x 6 miles= 113.04 square miles.
The density would be : 650000 people/ 113.04 square miles= 5750.17 people/square miles