because all parts of the cell work together
The value of a number increases when its subtracted by a negative. In the expression 5 - -4, since 4 is a negative integer that’s being subtracted, it’s added to the 5 — which would equal 9.
When you have two negative symbols, you have to multiplied them, so:
-*+ = -
You can to do more research lookin this: "Algebraic operations with symbols" or "The sign´s law"
I attached a illustrative image about this.
5 - (-4) (here is with -*- )
5 + 4