[tex]\text{Let's write it like}\\x+y=8\\+\:\:\:+\\z-x=6\\\downarrow\:\:\:\:\:\:\downarrow\\13\:\:\:\:8\\=>\\\displaystyle\\\left \{ {{x+z=13} \atop {z-x=6}} \right. \\\left \{ {{x+z=13} \atop {z=6+x}} \right. \\\downarrow\\x+x+6=16\\2x=7\\x=3,5=>z=9,5\\\text{Now we need to find y}\\3,5+y=8\\y=4,5\\\\\text{so we have}\\3,5+4,5=8\\+\:\:\:\:\:\:\:\:\:+\\9,5-3,5=6\\\downarrow\:\:\:\:\:\:\:\:\:\:\:\:\downarrow\\13\:\:\:\:\:\:\:\:\:\:8\\[/tex]
