you do that when you have a minus sign and a negative an example is like if you have 4-(-10)=? if you look at the part where its -(- it looks like a positive + so you add them so this is equal to 14, and when you are multiplying 2 negatives they both equal a positive an example is like -4 times -5 this equals 20, but if you have -5+-3=? this equals -8 because you are adding the numbers.
