$108 = 9 hours
$216 = x hours
we want to get to $216 from the original equation. one thing we can do to get there is to figure out what we need to multiply 108 by to get 216. to figure this out, we can calculate 216/108 = 2.
therefore, we can multiply both sides of the original equation by 2 to get
$216 = 9 * 2 hours = 18 hours as our answer