well, let's recall that A, B and C are medians of their segment, meaning each of those lines made by them are midsegments, Check the picture below.
[tex]CA = \cfrac{SR}{2}\implies 2CA=SR\implies 2(3x-1)=5x+4 \implies 6x-2=5x+4 \\\\\\ x-2=4\implies x =6~\hfill \stackrel{~\hfill CA}{3(6)-1\implies 17}[/tex]
