Verified answer

1) x=7-2y

3(7-2y)-4y=11

x=7-2y

21-6y-4y=11

x=7-2y

y=(21-11):10

x=5

y=1

(5;1)

2) 21x-14y=35

10x+14y=-4

31x=31

x=1

тогда 3*1-2y=5

-2y=2

y= -1

(1;-1)