The answer is 59,411.76 square miles
The population density is 170 people per square mile. In other words, in 1 square mile, there are 170 people. So, the question is in what area there are 10,100,000 people. Make a proportion:
170 people : 1 square mile = 10,100,000 people : x
After crossing the products:
x = 10,100,000 people * 1 square mile : 170 people
x = 59,411.76 square miles