Answers:
For each row, you're adding the values of 'a' and 'b'. The first row has 1+2 = 3.
When adding two negatives, add the positive versions of each number, then make the result negative. So -1 + (-2) can be thought of as 1+2 = 3, then you make everything negative.
When adding a negative to a positive, you'll subtract the positive version of each number. The final result is positive if the larger absolute value is positive, or otherwise it's negative. We have -4+1 that can be thought of as 4-1 = 3, but since the '4' is larger and it is negative, this means -4+1 = -3.
