Explanation:

We know that a negative value plus a positive value always count backward and the sign depends on the bigger value sign.
When you are adding a negative number to a positive number, then the value counts backward and takes the sign of the biggest number.
For example::
Adding -5 and +2
-5 + 2 = -3 (big number is 5 and its sign is '-')
Adding -3 and  +5
+5 + (-3) = 5 - 3 = +2(big number is 5 and it has sign '+').