B is the midpoint of segment AC trying to find AB using the definition of midpoint.

12

Step-by-step explanation:

A midpoint of a line means that each segment connecting from the midpoint to an end is equal. For this problem, this means that AB = BC, as B is the midpoint, and A and B are the ends. Therefore, we can say that:

AB = BC

2x + 6 = 5x - 3

add 3 to both sides

2x + 9 = 5x

subtract 2x from both sides

9 = 3x

divide both sides by 3

3 = x

Plugging 3=x into AB, this means that 2(3) + 6 = AB = 12