1.) How long will it take $2500 to reach a value of $8000 if it is earning 6.5% annual interest compounded continuously?

Since it is compounded continuously, I have to use the Pe^rt formula. It is asking me how long it takes for the task to be completed. How do I find the time?

2.) An equation that can be used to predict the population of a bird that is on the endangered species list is: Predicted population = Current population x 9^-(years/9)

The equation predicts the number of surviving birds over the next fifteen years if no measures are taken to protect the population.

It has been determined that if the population drops below 150 birds, it may not be possible to avoid extinction without removing the birds from the wild and raising them in captivity. Will the population fall below 150 in the next 15 years? If yes when?

On that question part, do I need to plug in 15 years or no?