When you have (for example) a positive number: 7, and a negative number: -10, you have 7 + (-10), which is just the same as 7 - 10, which is -3. When you have a positive plus a negative, it is always just the positive minus the absolute value of the negative (in other terms you have the positive minus the negative number with the negative taken off), and when you have a positive minus a negative, you are just adding the positive to the absolute value (once again, the negative with the negative taken off of it).

Here's a sample:

-19 + 21 = 2

21 + (-19) = 2

21 - (-19) = 21 + 19 = 40