Help, how do I write this problem?

Question

B is the midpoint between A and C.

9. a. Find x if AC = 3x - 1 and BC = 12 - x.

b. Use the answer for x to find the value of AB.

How do I write the problem to solve for x. Don't give me the answers just please explain it. Thanks so much. :D

Answer 1

since B is the midpoint, AC = 2BC

3x - 1 = 2(12 - x)

3x - 1 = 24 - 2x

5x = 25

x = 5

AB = BC = 12 - 5 = 7