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.
The answer is 2x+6
Explanation: 2 times 3 is 6 and 2 times x is 2x