1) y-2x=4
7x-y=1
y=2x+4
7x-y=1
y=2x+4
7x-(2x+4)=1
7x-2x-4=1
5x-4=1
5x=5
x=1
y=2*1+4
x=1
y=2+4
x=1
y=6
x=1
=> (1;6)
2) 3x-5y=1
x/4+11y/24=1
3x=5y+1
x/4+11y/24=1
x=5y/3+1/3
x/4+11y/24=1
x=5y/3+1/3
(5y/3+1/3)/4 +11y/24=1
5y+1/12+11y/24=1
(5y+1)*2+11y/24=1
21y+2=1*24
21y+2=24
21y=-2+24
21y=22
y=22/21
x=5(22/21)/3+1/3
y=22/21
x=110/63+1/3
y=22/21
x=110+1*21/63
y=22/21
x=131/63
y=22/21
=> (131/63;22/21)
3) 2*(3x-2y)+1=7x
12*(x+y)-15=7x+12y
6x-4y+1=7x
12x+12y-15=7x+12y
5x+12y-15=12y
5x=-12y+15+12y
5x=15
x=3
6(3)-4y+1=7(3)
18-4y+1=21
-4y+19=21
-4y=-19+21
-4y=2
y=-1/2
=> (3;-1/2)