On a number line, the coordinate of point A is negative and the coordinate of point B is positive.
The mid point is defined as:
a. When will the midpoint of line AB be positive?
When B>A
b. when will the midpoint of line AB be negative?
When A>B
c. when will the midpoint of line AB be zero?
When:
d. when will the distance from A to B be negative?
Never the distance is not defined as negative by definition of distance.
The distance is a magnitude and for this reason not makes sense say negative distance because is always 0 or positive, if for example we find the distance from A to A is 0 since we are in the same point, but we never say we travel -200 m to reach a city for example.