There are 42 students in a school lunch , 25 eat rice, 15 eat spaghetti and 11 eat neither rice nor spaghetti. (a) Draw a venn diagram to illustrate this information. (b) How many students eat both rice and spaghetti? (c) How many students eat at most one of the two types of food?

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