I honestly don't know what this is but I think it relates to factoring
I have a problem
9 - [(2-6) + (3-17)]
The answer is 27
But how?
If you like, you can think in terms of temperature.
If it was $\displaystyle 2^o$ and the temperature dropped by $\displaystyle 6^o$ then it will have fallen to $\displaystyle -4^o$
If it was $\displaystyle 3^o$ and it fell by $\displaystyle 17^o$,
it would be $\displaystyle -14^o$
$\displaystyle 9-[(2-6)+(3-17)]=9-[-4+-14]$
$\displaystyle -4$ means "subtract" 4
$\displaystyle -4+-14$ means "subtract" 4 and "subtract" 14.
That's subtract 18 altogether.
$\displaystyle 9-[-18]$ means "do the opposite" of subtract 18 from 9,
and since subtraction is the opposite of addition,
addition is the opposite of subtraction, then 9--18 is 9+18.
To get the hang of things, you can think that way.
-- is + because
If you take a positive # c
c
and you subtract a negitive number d
c-(-d)=r
You are taking away a bit less than nothing.
And if you are taking away less than nothing r must be moving further from nothing.
Like owning money.
I owe 5$ and someone subtracts 4 dollars from my debt, now I owe 1$
I.E.
-5--4=-1
idk if that helps just trying to put usefull info out there