Ok, so
Here we have the function:

Now we want to find the tripling-time for this population of deer.
If we make t=0, we will find the initial population of deer. This is:

Now, we want to find the time "t" such that this population is the triple.
This is:

We got this exponential equation:

As the base is the same, we could equal the exponents:

Therefore, tripling-time for this population of deer are 3 years.