
Answer:
the initial population P₀ was  5938 people
Step-by-step explanation:
since the rate of increase of the population is proportional to the population itself, then the corresponding equation for a population P and time t is
dP/dt = k*P , where k= proportionality constant
dP/P = k*dt
∫dP/P =∫ k*dt
integrating between time t=0 (with P=Pâ‚€) and time t=t (with P=P)
ln (P/Pâ‚€)=k*t
if the population doubled , so P=2*Pâ‚€ at t=5 years then
ln (2*Pâ‚€/Pâ‚€)=k*5 years
k = ln 2 / 5 years
then the population is P=9000 for t= 3 years
ln (9000/Pâ‚€)=k*3 years
ln (9000/Pâ‚€)=ln 2 / 5 years *3 years
solving for Pâ‚€
P₀ = 9000 people * e^ (- ln 2 / 5 years *3 years ) = 9000 people *2 ^(-3 years / 5 years) = 5937.78 people ≈  5938 people
therefore the initial population P₀ was  5938 people