[tex]\dfrac{x}{z} = 0,(6) \Rightarrow \dfrac{x}{z} = \dfrac{6}{9} \Rightarrow x = 6 k,\quad z = 9k, ~~ k\in \mathbb{R}^* \\ \\ \dfrac{x+z}{2x+z} = \dfrac{6k+9k}{2\cdot 6k + 9k} = \dfrac{15~\cdot \not{k}}{21~\cdot \not{k}} = \dfrac{15}{21} = \dfrac{5}{7}[/tex]
