if we take a look of the graph, let's notice that
AC = 4x + 4
AB = 14
BC = 3x - 4
now, AC = AB + BC,
[tex]\bf AC=AB+BC\implies AC-BC=AB\implies \stackrel{AC}{(4x+4)}-\stackrel{BC}{(3x-4)}=\stackrel{AB}{14} \\\\\\ 4x+4-3x+4=14\implies x+8=14\implies x=6 \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ AC=4(6)+4\implies \boxed{AC=28}[/tex]
