Question 387478
B is between A and C. 
AB=2x-4 
BC=7x+5 
AC=82 

This is like segment addition in Geometry.

AB+BC=AC
(2x-4)+(7x+5)=82   Combine your like terms
9x+1=82  Subtract 1 from both sides. This will cancel your (+1)
9x+1(-1)=82(-1) Simplify
9x=81  Divide both sides by 9

x=9