Prove that NEGATIVE times NEGATIVE gives POSITIVE - Real-life example

You shall know that:

+*+ = +
+*-  = -
-*+  = -
-*-  = +

But why?

Here is a real-life example.

Video:
Prove that Negative Times Negative Gives Positive (youtube.com)

Example 1: + +

 Let's say you gain 2 dollars every day - the coefficient is +2
How much the amount of money that you have would change in 3 days? - the number of days is +3
So the amount of money you earned in +3 days with +2 each day is 3*2 = +6 dollars

 

Example 2: - +

How much money did you have 3 days ago comparing to now? -3 days
 -3 days, times +2 dollars per day, -3*2 = -6 dollars

Example 3: - -

You spend 2 dollars each day - the coefficient is -2
How much money, comparing to now, you had 2 days ago? -2 days
-2 dollars per day * -2 days ago, -2*-2 = +4 dollars