When you are dealing with this type of situation, there are some strategies that you can try and that are likely to help you get out of your problem.
First, if you have people around you that might help you out, take advantage of it. Try speaking with an instructor or classmate and return to the question afterwards. Another strategy is to look over the question again and try to find any hints that address the part of the calculation that you are unsure about. Finally, if these strategies do not work, submit your answer and adjust it according to the feedback you receive.