Question 1053040
x + y + z = 12
x + 2y + 3z = 31
9y + z = 1 ----> z = 1 - 9y


Substitute the expression for z into the first two equations.


x + y + (1 - 9y) = 12 ---> x - 8y = 11
x + 2y + 3(1 - 9y) = 31 ---> x - 25y = 28


Subtract the equations to get


x - 8y = 11
-x + 25y = -28
----------------------
17y = -17
y = -1


9y + z = 1
9(-1) + z = 1 ---> z = 10


x + (-1) + 10 = 12
x = 3


solution: (x, y, z) = (3, -1, 10)