Hey there!
If Owen has 12 pieces of rope that are 4 yards long each, or 12 feet long (a yard is three feet in length), he has 144 feet of rope total. If he uses 6 pieces of rope that are 6 feet long, he has used 3 pieces of his 12 foot long rope pieces, since 6*6 and 3*12 are both equal to 36 feet of used rope.
All you need to do is subtract that 36 feet from the total of 144 feet to get 108 feet remaining.
Hope this helped you out! :-)