Item 1.
The initial condition is \(f(0) = 3\text{.}\) To get \(f(n+1)\) we would double the number of snails in the terrarium the previous year, which is given by \(f(n)\text{.}\) Thus \(f(n+1) = 2f(n)\text{.}\) The full recursive definition contains both of these, and would be written,
\begin{equation*}
f(0) = 3;~ f(n+1) = 2f(n)\text{.}
\end{equation*}