simultaneous equations

1)
2x+y=2.....(1)
x-y=1........(2)
From (2),
x=y+1.....(3)
Sub (3) into (1)
2(y+1)+y=2
2y+2+y=2
y=0
Sub y=0 into (3)
x=0+1
x=1
So x=1, y=0

2) 4x+y=x+y-6=-5
Rewrite the simultaneous equations as follows:
4x+y=-5.....(1)
x+y-6=-5.....(2)
(1) - (2):
3x+6=0
x=-2
Sub x=-2 into (2)
(-2)+y-6=-5
y=3
So x=-2, y=3

3)7x-4y-1=5x+2y+9=0
Rewrite the simultaneous equations as follows:
7x-4y-1=0.....(1)
5x+2y+9=0.....(2)
(2)*2:
10x+4y+18=0...(3)
(1) + (3):
17x+17=0
x=-1
Sub x=-1 into (2)
5(-1)+2y+9=0
-5+2y+9=0
y=-2
so x=-1, y=-2

樓主條式有少少打錯，我估應該係我所估的這樣。