a. Add 13 and 10:
As they have the same sign (both positives) you have an addition:

b. Add (-8) and (-12):
As they have the same sign (both negatives) it is an addition too (the result will have the same negative sign):

c. Add (-5) and 4:
As they have different signs, (one negative and one positive) it is a subtraction, the result will have the sign of the greates number:

d. Add 7 and (-3):
As they have different signs (one positive and one negative) it is a subtraction. The result has the sign of the greatest number:
