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.
