When adding and subtracting negative numbers, the sign of the number needs to be taken into account.
The simplest way to deal with this is to turn the second number into a positive number. But changing the sign of the second number also means changing the sign of the operation.
If the operation was a subtraction, then it becomes an addition: if it was an addition, then it becomes a subtraction.
1. Calculate +5 - -3.
Change -3 to +3, and because the sign of the second number has changed, change the subtraction to an addition. Then 5 + +3 = 8.
2. The temperature in Bristol is -4ºC. It increases during the day by 7ºC. What is the new temperature?
-4 + +7 is -4 + 7.