Question

What is the length of BC¯¯¯¯¯ ?

Answer 1

X + 9 = 2x - 8
2x - x = 9 + 8
x = 17

BC = x = 17

answer
17 units

Answer 2

You would set AC and AB equal to each other to find out what x is. 

x+9=2x-8
x+17=2x
17=1x
17=x

Since CB is just x, CB equals 17.