LOL
For debt you can't multiply Visa with Master, you have to add them together wich gives the total debt.
To understand why a minus multiplied by a minus becomes positive just look at the math language on both sides of the equal sign.
Visa = -$400
Means you have a debit in your account of $400.
So to state it the other way, multiply with -1 on both sides of the equal sign.
-1 x Visa = -1 x -$400
-Visa = $400
Means your account is still in debit by $400.
LOL, I have used a lot of math in my Engineering career, but at school it was the only subject I did not have to try and learn, it just happened automatically. Did not go to school in the US though.