If AB = BC then you can solve for x which might a good first step. From the givens and the way the diagram is marked, you have
AB = BC
x + 6 = 2x + 12 This is an impossible condition. Using 3x - 31 doesn't help very much
x + 6 + 2x + 12 = 3x - 31
3x + 18 = 3x - 31 From which (if you subtract 3x from both sides
18 = - 31 which never going to be true.
Let's go back to AB = BC
x + 6 = 2x + 12 Subtract x from both sides.
6 = 2x - x + 12
6 = x + 12 Subtract 12 from both sides.
6 - 12 = x
x = - 6
When you check this result, you get
x + 6 = -6 + 6 = 0
2x + 12 = 2*-6 + 12 = 0
X = -6
AB = 0
BC = 0
AC = 0