1.7х + 2у = 9    *(-1)
   5х + 2у =11

   -7x-2y=-9
   5x+2y=11

   2x=-2
   5x+2y=11

    x=-1
    5*(-1)+2y=11
    x=-1
    y=8

2. 9х-2y=-17 I*(-1)
x-2y=7
-9x+2y=17
x-2y=7
-9x=17
x=7
-8x=24
х=3 
3 -  2y = 7
-2y = 7-3
-2y = 4
y = 4/(-2)
y = -2

3. x+7y=19
x+5y=13 I*(-1) 
x+7y=19
-x-5y=-13
x + 7y - x - 5y = 19 - 13 
2y=6
y=3
x+21=19
x=19-21
x=-2

4. 5x-2y=15
2x-y=7  I*(-2)
5x-2y=15
-4x+2y=-14

5x-2y+4x+2y=-14+15
x=1

5*1 - 2y=15
-2y=15-5
-2y=10
y=-5