(-1) - (-9)
Think of negative numbers as COLD numbers (the freezer has a negative temp)
an positive numbers are HOT numbers.
Here you have a temp of -1 you want to take away some cold (because -9 is a cold number)
If you take cold out of a room then the room gets warmer!
So take away a negative can replaced by adding.
Taking away some cold air has the same effect as adding some warm air.
Both times the room gets warmer. Do you get it?
- - = + + = +
(-1)-(-9) = (-1) + (+9) = -1+9 = 8
(-1)-(-9) = -1 - -9 = -1+9 = 8