The previous population was 14000.
Explanation
The population decreased by 35% since the last measurement. So the current population is less than the previous population.
current population=9100
previous population =x
current population is 35% less than the previous population x
The previous population was 14000.
I would start by setting up a chart like I did below.
Label one column age now and the other age in 5 years.
Since we don't know the son's age we use x.
We do know that the man's age is 4 times the son's age.
So the man's age will be 4x.
In the age in 5 year column, we add 5 to their current ages.
Now set up our equation.
Since it says "in five years" we use information in second column.
In 5 years time, he, "4x + 5", will be, equals,
3 times as old as his son, "3(x + 5)".
So we have 4x + 5 = 3(x + 5).
Solving from here, we find that x = 10.
So the son is 10 and the man is 4 times his age or 40.
