Let \[f(x) =
\begin{cases}
3x + 5 &\text{if }x<-3, \\
7-4x&\text{if }x\ge -3.
\end{cases}
\]Find $f(5)$.
Since $5$ is greater than or equal to $-3$, I use the second piece: $f(x) = 7-4x$. Plugging in $x=5$, I get $f(5) = 7-4(5)$. Simplifying, I obtain $f(5) = 7-20 = -13$. 