When you multiply two numbers, let’s say x and y, there are no negatives, so you know that it is positive. So that means it’s xy. Now, imagine we make that a -x times y. Since there is only 1 negative, it is negative. So that would be -xy. Now, imagine you have -x times -y. Since there are an even number of negatives, they cancel out!! (Magic). So -x times -y is xy.