How to add negative and subtract positive numbers

You should ask this to Khan academy they will give you a detailed explanation of this subject.

Do you want to add two negative numbers together? If so, then you basically add them as if they were positive and then add in the negative sign.                              -a-b    -(a+b)
If you want to add a negative number to a positive number, then you're basically subtracting it from the positive number, because you multiply + by -, which is -.
 a+(-b)   a-b
I explained this the best I could, since the question isn't exactly clear, but like in the other answer, Khan Academy could help.