There are six weeks that we average to get our final grade but our final exam is 15% of the final grade so what would be the formula to use?

If the final exam is worth 15%, then the six weeks must be work 85%.
Take your percentage for the six weeks, and multiply it by 0.85. Then, take your percentage for the final exam and multiply it by 0.15. Add the 2 numbers together, and that'll be your final grade.
Example: 92% over the six weeks, 80% on the final.
92(0.85) + 80(0.15) = 78.2 + 12 = 90.2, or a 90.2% as the final grade.
(If you want to know what you need on your final exam in order to get a certain grade in the class, set your equation equal to that percentage (i.e. 90 = A) and solve for your final exam score)
Take your percentage for the six weeks, and multiply it by 0.85. Then, take your percentage for the final exam and multiply it by 0.15. Add the 2 numbers together, and that'll be your final grade.
Example: 92% over the six weeks, 80% on the final.
92(0.85) + 80(0.15) = 78.2 + 12 = 90.2, or a 90.2% as the final grade.
(If you want to know what you need on your final exam in order to get a certain grade in the class, set your equation equal to that percentage (i.e. 90 = A) and solve for your final exam score)

Find the total number of points possible for the class and work from there. If there are 100 points possible, the final would be worth 15 points and the previous 6 weeks would have been worth 85 points.