a) x + 2y = 20 | x3
3x + 6y = 60
y + 3z = 30 | x3
3y + 9z = 90
3y + 9z = 90
3x + 6y = 60
------------------------(+)
3x + 9y + 9z = 150
b) 8x + 6y - 6z =
2(4x + 3y - 3z)
x + 2y = 20 | x2
2x + 4y = 40
2x + 4y = 40
y + 3z = 30
-------------------(-)
2x + 4y - ( y + 3z ) = 10
2x + 4y - y - 3z = 10
2x + 3y -3z = 10 | x2
2(4x + 3y - 3z) = 20
8x + 6y - 6z = 20
