======================================================
a3=a1+2d=0
a8=a1+7d=25
система
a1+2d=0
a1+7d=25
a1=-2d⇒-2*5=-10
5d=25⇒d=5
a6=a1+5d=-10+25=15
<span> 5x-y=17 y=5x-17</span>
<span>-9x+8y=19 -9x+8(5x-17)=19 -9x+40x-136=19 31x=155 x=5 y=5*5-17=8</span>
<span>x=5 y=8</span>