segment addition postulate says
AB+BC=AC
assuming that A, B, C are in order, and assuming that those are the points,
legnth of AC=|A-C|=39 (|x| means absolute value of x)
A=2x-8
b=17x
AB=|A-B|=|2x-8-17x|=|-8-15x|
we are only given AB but not BC so this is not solvable unless you know BC
AB+BC=AC
BC=AC-AB
BC=39-|-8-15x|
that's as far as I can go since we don't know any more