1.
7х+2у=9,
5х+2у=11,
-7х-2у=-9,
5х+2у=11,
-7х-2у+5х+2у=-9+11,
-2х=2,
х=-1,
у=(11+5)/2=8 ⇒ (-1; 8),
2.
9х-2у=-17,
х-2у=7,
9х-2у=-17,
-х+2у=-7,
9х-2у-х+2у=-17-7,
8х=-24,
х=-3,
у=(-7-3)/2=-5 ⇒ (-3; -5),
3.
х+7у=19,
х+5у=13,
-х-7у=-19,
х+5у=13,
-х-7у+х+5у=-19+13,
-2у=-6,
у=3,
х=13-5*3 = -2 ⇒ (-2; 3),
4.
5х-2у=15,
2х-у=7,
5х-2у=15,
-4х+2у=-14,
5х-2у-4х+2у=15-14,
х=1,
у=2*1-7=-5 ⇒ (1; -5)