Помогите 17 и 19. 10 ьаллов

$\left \{ {{3^{x-1}=9^y} \atop {2x-y=5}} \right.$
$\left \{ {{3^{x-1}=(3^2)^y} \atop {2x-y=5}} \right.$
$\left \{ {{3^{x-1}=3^{2y}} \atop {2x-y=5}} \right.$
$\left \{ {x-1=2y} \atop {2x-y=5}} \right.$
$\left \{ {x=2y+1} \atop {2x-y=5}} \right.$
$\left \{ {x=2y+1} \atop {2(2y+1)-y=5}} \right.$
$\left \{ {x=2y+1} \atop {4y+2-y=5}} \right.$
$\left \{ {x=2y+1} \atop {3y=3}} \right.$
$\left \{ {x=2y+1} \atop {y=1}} \right.$
$\left \{ {x=2*1+1} \atop {y=1}} \right.$
$\left \{ {x=3} \atop {y=1}} \right.$
$x-y = 3-1 = 2$
ответ: A

19. $a=4b$
$c+3b = 0$
$c = -3b$
$\frac{a}{c} = \frac{4b}{-3b} = - \frac{4b}{3b} = - \frac{4}{3} =-1 \frac{1}{3}$
ответ: D