States that if there are p ways to do a thing, and q ways to do another thing, and these two things are independent, there are p*q ways to do both things.
One object from each set:
2 from one set, 5, 6 and 7 by others. Sets are independent, so, by the fundamental counting principle:
The commutative property of addition means we can add two integers in any order. So yes, It would still apply to two negative integers (for example, -2 + -3 and -3 + -2 both equal five)