The population density of a place is defined as follows:

A. Let us calculate the density of San Diego . The population is 1356000 and area 372.4. So the density is

In the case of Los Angeles we get

So, it is clear that Los Angeles has a greater density than San Diego.
B. We have the following equality for Sacramento. Let x be the population of Sacramento. Then

So we need to solve this equation for x. To do so, simply multiply by 100.1 on both sides, so we get

So population of sacramento is about 479690 people.
C. In here, we are missing the area. So let x be the area of San Francisco. We will have the following equation

So, we multiply both sides by x. We get

Finally, we divide both sides by 17855. So we get

So the area of San francisco is about 46.90 squared miles.