Answer
(a) $63$ years
(b) $100$ years
Work Step by Step
(a) Given $n_0=7.1, r=0.011$, use the exponential model we have $n(t)=7.1e^{0.011t}$
For the the population to double, $n(t)=2n_0$, so $2n_0=n_0e^{0.011t}$ and we get $t=ln2/0.011\approx63$ years
(b) For the population to triple, $n(t)=3n_0$, so $3n_0=n_0e^{0.011t}$ and we get $t=ln3/0.011\approx100$ years