to put simple like this
add as normal
2+2=4
add if were positive but keep it negative
-2+-2=-4
at this point it basically turns in to a subtraction problem
2+-2=0
here is another way to look at it. two positives (+) and two negatives (-)
+ +
- -
if there is a negative under it, it is canceled out.
in this there are 5 positives and 3 negatives. each negative will cancel one positive leaving +2 left
+ + + + +
- - -
hope that explains it some what.