Answer:
12 Years
Explanation:
216 - 168 = 48 (new deer every 3 years)
48 / 3 = 16 (new deer every 1 year)
Since we end at the year 2009, we need to start counting from 312 deer.
500 - 312 = 188 (Needed new deer)
188 / 16 = 11.75 (Years needed)
11.75 rounded to the year (whole number) is 12.