ответ: 7 - 2 = 5

[tex]\displaystyle \left \{ {{3x-4y=13} \atop {x+y=9}} \right. \\\\\left \{ {{3x-4y=13} \atop {x=9-y}} \right. \\\\3(9-y)-4y=13\\y=2\\x=9-2\\x=7\\(x,y)=(7,2)[/tex]