The domain is talking about the "range" of the horizontal axis therefore you will be focusing on the x-intercepts.
The answer will be All non-negative real numbers less than or equal to 18
because the x-intercepts lies at 0 and 18. The answer makes sense because the furthest you can go is 18 ft and the closest you could go is 0 ft. The "all non-negative real numbers" puts a restriction on the least distance it could travel so that means that it stops at 0 ft because if you go any further, you will end up in the negatives and it clearly states "non-negative".
