The function that relates the speed of the sailboat, $f(x)$, in knots to the length of the sail, $x$, in feet is:
$\leadsto\sf\textbf\:f(x)\:=\:2\sqrt{x}$
$\leadsto\sf\textbf\:g(x)\:=\:\text{function not provided}$
The function that relates the speed of the sailboat, $g(x)$, in knots to the length of the sail, $x$, in feet is not given in the question. Therefore, we cannot provide a mathematical expression for $g(x)$ without additional information.



