The population in y years is 307,650 plus the number of years that has pased since she had 9 year: (y-9) multiplied by 850 wich is how much the population increases in a year.
We can simplify the problem eliminating the parenthesis:
126/10 would mean that from the original number, the decimal point is at 126.0 so when it is divided by 10, it will move over one placement, making it 12.6... I would estimate that your answer is C. 12.6