Points A, B, and C are collinear, then:
AB + BC = AC
Point B is the midpoint of AC, then:
AB = BC
Replacing with the equations of AB and BC, we get:
2x + 16 = 4x - 4
2x is adding on the left, then it will subtract on the right
4 is subtracting on the right, then it will add on the left
16 + 4 = 4x - 2x
20 = 2x
2 is multiplying on the right, then it will divide on the left
20/2 = x
10 = x
Then:
AB = 2x + 16 = 2(10) + 16 = 20 + 16 = 36
BC = 4x - 4 = 4(10) - 4 = 40 - 4 = 36
AC = AB + BC = 36 + 36 = 72