In High School, I had trouble adding positive and negative numbers because I didn't understand the concept of positive and negative numbers in addition. But any positive number plus the same negative number equals 0. Now, I understand how to add both positive and negative numbers. I just used the number line in my head as a calculator. For example, -10+9=-1. Or, -11+17=6. If your adding a negative number with a positive number, then your going right on the positive number line. If your adding a positive number with a negative number, your going left on the negative number line. One more example, -7+-7= =-14. If your adding two negative numbers together, then your answer will be an increased negative number.