When you add 2 negatives you get a negative for example -5+-15= -20 when you add a positive to a negative it increases the number for example -20+7= -13. Hope this helps
A negative plus a negative is always a negative number, (same concept for positives) so adding a positive and a negative is like subtraction, for example 10+-7, is basically 10-7 which is 3. But 11+-13 would be 11-13 which is -2.
