First we need to know how many months is 2 years. To answer that we multiply the number of months in a year by 2 and get 48 months.
Every 3 months the population of turtles doubles, but to get how many times the population doubles, you need to divide 48 months by 3 to get 16.
So population of 400 will get <u>doubled</u> 16 times in the following <u>two</u> years.
We construct the following equation where is the final population,
Hope this helps. :)