Answer:
Since the amount of taxes collected is proportional to the population of the city, we can write the relationship between the amount of taxes and the population as:
T = kP
where T is the amount of taxes, P is the population, and k is the constant of proportionality.
From the information given, we can find the value of k:
4 billion = k * 2 million
k = 4 billion / 2 million = 2
So, for the population of 3 million in 2017, the amount of taxes collected would be:
T = k * P = 2 * 3 million = 6 billion
So, in 2017 the city collected 6 billion dollars in taxes.