To me, "simplify 6(15-3)+4-6.4" involves simplifying the left and right terms first and then adding the two results together; perform the operations as far as it is possible to perform them.
6(12) + 4 - 6.4 (Note that the 15-3 inside parentheses must be done first.)
72 + (-2.4)
72 - 2.4 = 69.6 (answer)