There are two main reasons a correct answer may not be accepted in an exercise (even if it is the same answer given in the hints):
1. The answer entered isn't really the same as the answer in the hints.
2. 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. The article below deals with the second kind of issue.
Problems like this one should be reported by the Report a mistake in this question-link on the lower right of the exercise page after checking that your answer is correct.
Thank you for your patience and understanding.
September 7 2016