The specific numbers did not appear in the problem. The general method is:
Given `(dp)/(dt)=kp` where `k` is a proportionality constant and `p` is the population at time `t` . We know that any function whose rate of change is proportional to the function itself has the following property:
`(dy)/(dx)=ky=> y=Ce^(kt)` with k the proportionality constant and C a constant.
Thus we have `p=Ce^(kt)` as the population function. You will take the two data points (the population at two different times) to solve for the two unknowns C and k using substitution.
Then with that function you can evaluate at any other time t.
In order to find when the population triples, you will solve for t given a known p.