To solve this, we just need to find the distance between the points A and B in the number line, and then, divide that distance by 2.
Remember that the distance formula between tow point in the number line is:
From our point we can infer that
and 
, so lets replace the values:
Now, to find the coordinate of the midpoint, we just need to divide that distance by 2:
We can conclude that the coordinate of the point M, the midpoint of AB, is 4
Remember that the distance formula between tow point in the number line is:
From our point we can infer that
Now, to find the coordinate of the midpoint, we just need to divide that distance by 2:
We can conclude that the coordinate of the point M, the midpoint of AB, is 4