The prob is basically asking approximately how many people does the population increase in 1 day.
We have given the rate of people born in every 2 seconds. Rate of born = 9 people per 2 seconds.
We have also given the rate of people die in every 2 seconds. Rate of death = 3 people per 2 seconds
So the number of people live/increase in 2 seconds is = (9-3) = 6
i.e. 3 people will be living in 3 seconds.
Now we have to calculate the number of seconds in 24 hours.
24 hours = 24*60*60 seconds.
Population increase in 1 day = 24*60*60*3 = 259,200