-13+9=-4
When you add a positive number to a negative number the answer will get closer to 0 and then progress up in positive numbers. In this case we are finding which of the numbers equals -4 when added to -13. Remember, if you add a negative to a negative you will get a negative.
-13+-17=-30 This is the wrong answer.
-13+-9=-21 This is the wrong answer.
-13+17=4 This is the wrong answer. (This is positive 4 we are trying to find -4)
-13+9=-4 This is the right answer
Therefore X=9
