computing amount of principal after number of years, math homework help

A mathematical model for the Future Value of a savings account earning interest that iscompounded continuously is given by the equation FV = Pert, where FV is the amount after t years,P is the principal amount invested at t = 0, and the principal is assumed to grow continuously at arate, r. How many years will it take the principal to triple if the annual rate is 12%?  Please explain how you arrived at your answer. Thanks!