6) 3x-5y = 8
x = -y
3x-5y = 8
x + y = 0
3x-5y = 8
3x + 3y = 0
-8y = 8, y = -1
x=1
7) 3(x-y) + 5x = 2(3x -2)
4x -2(x+y) = 4-3y
3(x-y) + 5x = 2(3x -2)
4x -2x-2y+3y -4 = 0
3x-3y + 5x = 6x -4
2x+y -4 = 0
2x-3y+4 = 0
2x+y -4 = 0
-4y +8 = 0, y = 2
x=1
8) 2 - 5((1/5)y -2x) = 3(3x+2) + 2y
4(x-2y) - (2x +y) = 2 - 2(2x+y)
2 - y +10x = 9x+6 + 2y
4x-8y - 2x -y = 2 - 4x-2y
-3y +x -4 = 0 | *6
6x-7y-2=0
-18y +6x -24 = 0
6x -7y -2 =0
11y +22 = 0, y = -2
x =-2
9) 2x + y -8 = 0 | *3
3x + 4y -7 = 0 | *2
6x + 3y -24 = 0
6x + 8y -14 = 0
5y + 10 = 0, y = -2
x = 5
10) 3x - 4y -2 = 0
5y -x -6 = 0 | * -3
3x - 4y -2 = 0
-15y +3x +18 = 0
-11y + 20 = 0, y = 20/11
3x -4*20/11 -2 = 3x -7 3/11 -2 = 3x -9 3/11 = 0
x = (102/11)/3 = 34/11 = 3 1/11
Последнее стоило бы перепроверить.