The equation is $w = 6m + 9$, where $w$ is the total weight in ounces and $m$ is the number of cookies.

When $m = 0$ (no cookies in the carton):

$w = 6(0) + 9 = 9$

This means the carton weighs $9$ ounces when empty. Therefore, $9$ represents the weight of the empty carton.