Do not entertain the answers that tell you to “just memorize that ‘minus times minus is plus,’” and so on. Think of a number line with 00 in the middle, and the numbers 11, 22, 33, and so on to the right, and some special numbers 1¯, 2¯, 3¯, and so on to the left. I’ll explain what those bars mean in a moment.

https://www.quora.com/If-a-negative-times-a-negative-is-a-positive-why-isnt-a-positive-times-a-positive-a-negative/answer/Robert-Smith-9