Answer:
320,000 people / km²
Explanation:
We need to find the area of the city first.
The area of a triangle is:
, where b is the base and h is the height.
Here, both b and h are the legs of the right triangle and they're equal to 25 km. So:

km²
Population density is simply (population) ÷ (area). Here, the population is 100,000,000 people and the area is 312.5 km², so:
100,000,000 ÷ 312.5 = 320,000 people / km²