My maths teacher used a rather helpful little memorable explanation for my class (honestly that guy was a fucking legend, wish I had the time to pop into my old school for a quick visit with some of those teachers, some really are damn fine at their jobs)

See the positives and negatives as cowboys. It’s a silly thing but it’s the silly stuff that sticks in your head far easier and longer. For below, take a to mean a cowboy and b to mean a town.

A good cowboy visiting town is good: a + + b = positive (or just a + b)

A good cowboy leaving town is bad: a + - b= negative (or just a - b)

A bad cowboy visiting town is also bad: -a + b = negative (shown as a - + b)

But, a bad cowboy leaving town is good: -a - b = positive (shown as a - - b)

Now I’m not a teacher (don’t envy their job at all) and it has been… let’s just say quite a few years since I was at school and leave it at that… if I’ve not explained it clearly then my apologies, but that should hopefully be useful. It’s also good to see positives and negatives as literal mirrors of each other. Zero is the mirror line, but they are the exact same and work the same way… so remove the positive and negative prefixes, you’ve just got numbers… and adding and subtracting then becomes trivial. The + and - complicate the look of it… but ultimately the numbers behave the same regardless if they’re positive or negative so they only way to make add and subtract behave in the exact opposite way to what they typically do… is to use numbers from the opposite side of that mirror line. Otherwise add will always go up, and subtract will always go down…