The unusually bright nineteenth century painting was well-known at the art museum.
Explanation:
The hyphen is used correctly when it joins two words that should act as a single adjective. However, in addition, the two words joined by the hyphen must be placed before a noun so that the use of the hyphen is really correct.
An example of this can be seen in the phrase "The unusually bright nineteenth century painting was well-known at the art museum" where the words "well-known" represent a single adjective and are positioned before the noun "art museum".
