Verified answer

ax+by=8

A(1;2) - х=1, у=2

В(2;-4) - х=2, у=(-4)

подставляем в функцию "иксы" и "игреки", получаем след. систему: [tex]\displaystyle \left \{ {{a+2b=8} \atop {2a-4b=8}} \right. \\\\ \left \{ {{a=8-2b} \atop {2(8-2b)-4b=8}} \right. \\\\ 16-4b-4b = 8 | :8\\\\ 2-b=1 \\\\ -b=1-2\\\\ \left \{ {{b=1} \atop {a=6}} \right.[/tex]

b-a = 1-6 = -5