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.
