a. -6 - 7 = -6 + -7 = -13. Choice A is wrong b. -9 + 8 = -1. Choice B is wrong c. -36 + 8 = -26. Choice C is wrong d. 1/2(-6 - 6) = -6. Choice D is correct.
Answer (13 of the original arrangements are duplicates now, as
AAAF F F FDDDDCC, AAF F F FDDDDCCA, ..., CAAAF F F FDDDDC
are identical): 900900/13 = 69300.