In a small town the stray dog is rapidly increasing there are currently 15 stray dogs, and is estimated that the population will triple every year

If there are 15 stray dogs the first year, and it is estimated that the population will triple every year, the first year you will have 15 stray dogs, 2nd year you will have 45 because you are multiplying 15 by 3. The 3rd year you will have 135 dogs because you are multiplying 45 by 3. And so on

The answer to that is 6