Answer: So, if m divides n then n/m = x, and x is integer.
then -n/m = (-1*n)/m = -1*n/m = -1*x = -x.
So if x is integer, -x also is integer, then -n/m is integer and then m divides -n.
Where you used that in the integers set each number a has a opposite (also in the set ) such that a + b= 0, and b = -a = -1*a.
