Segment addition postulate. trying to my son figure out how to solve. given ac=39. a=2x-8, b=x×17. find x, ab and bc. we got 25 for x, and 42 for both ab, bc. its not correct. 42+42 does not equal 39.

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