Answer:
3
Explanation:
The midpoint of two numbers on a number line can be calculated by finding the sum of the two given numbers and dividing that sum by 2. The resulting answer will be the midpoint of the given numbers.
In other words we can say that midpoint is the average of the given numbers. The given numbers are -9 and 15. The sum of these two numbers is:
-9 + 15 = 6
Half of 6 is 3, so 3 is the midpoint of -9 and 15 on the number line.
In order to verify you can calculate the difference from 3 to 15 and from 3 to -9, the result will be the same.