Let's see it...
-x+0+(-x)=0 ???
There is a + before the parenthesis, the sign is the same.
-x+0-x = 0
Since the x have equal sign, we add them to get -2x
We have 2x - 0 = 0
Move the zero to the right, so 2x = 0
Divide both for 2...
x = 0
Substitute 0 to x, so -0+0+(-0) = 0
0 = 0 the statement is true