The sigma means that you add up all the values you get after you plug in the designated range of n.
so, what that means in simplest terms:
plug in the value for n on the bottom first, which is 2. then plug in everything up to the number on top of the sigma
so you'd plug in, 2 for n, then, 3, then 4, then 5, then 6, then 7, and finally 8 into the expression -(n-5)
doing this we'd end up with:
-(2-5) = 3
-(3-5) = 2
-(4-5) = 1
-(5-5) = 0
-(6-5) = -1
-(7-5) = -2
-(8-5) = -3
so then, after we do all that, we add up all the values we found, in this case they all cancel each other out and go to zero:
3 + 2 + 1 + 0 + (-1) + (-2) + (-3) = 0