What are the rules for adding and subtracting negative and positive numbers?

Adding positive numbers....just add as usual,,,,the answer, of course, will be positive

Adding negative numbers....forget the signs......add as you would two positives......go back and append a "negative" sign to the answer....Example   -21 +  (- 13 )   =  -21 - 13.....Add 21  + 13  = 34     .....append the negative sign to the result   = -34

Numbers with different signs.....this usually causes the most trouble.......here's the "trick'  :

Forget the signs.......take the "smaller" number and subtract it from the "larger"......in the answer, use the sign that was on the "larger" number

Ex :

-21  + 6

Forget the signs

Take the "larger" number, 21, and subtract the "smaller" number

21 - 6   = 15

Now.....use the sign that was originally on the "larger" number.....this is a negative......so....the aswer is :   - 15

One more note....subtracting a negative turns it into a positive

Ex :

-15  - (-9)   =   -15 + 9     and now.....we just use the last rule

15  - 9   = 6   ....and since the "larger" number  was negative......the answer is - 6

Hope that helps......