Subtracting a negative number is the same as adding the positive.
The answer may be positive or negative depending on the size of the numbers.
Work from left to right to get the answer.
Examples
15 - -21 - 5 = 15 + 21 - 5 = 36 - 5 = 31
-117 - -30 - -21= -117 + 30 + 21= -87 + 21= -66
Remember
-11 + 3
is the same as
3 + -11
which is the same as
3 - 11
which is -8