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.