Because B is the midpoint of AC, point B divides AC into two equal segments:
AB = BC
Substitute all known values into the above equation:
x + 5 = 2x – 11
Add 11 to both sides of the equation:
x + 16 = 2x
Subtract "x" from both sides of the equation:
16 = x
We have proven that x = 16. Now we are asked to find the measure of AB. Simply substitute the known value of "x" into the equation for AB:
AB = 16 + 5
Simplify the right side of the equation:
AB = 21
The measure of segment AB is 21
I hope this helps!