There are two main reasons a correct answer may not be accepted in an exercise (even if it appears to be the same answer given in the hints):
- The answer entered isn't really the same as the answer in the hints.
- The answer in the hints is incorrect.
The first issue is related to formatting your answer, so double check this article to make sure the answer entered matches the formatting. This article deals with the second issue.
Problems like this should be reported using the Report a problem question-link on the lower right of the exercise page after checking that your answer is correct.