Answer: 6 < x < 14
This is when the curve goes downhill as we move to the right. We don't include either endpoint.
The inequality 6 < x < 14 converts to the interval notation (6, 14) which is not to be confused with ordered pair notation. Unfortunately the two notation types look identical. In my opinion, the inequality 6 < x < 14 is more descriptive, so I'd stick with that answer format.
Side note: the increasing portion of the graph happens when 0 < x < 6.