x + y - z = 9
3x - y + z = -13
____________(+)___
4x = -4 => x = -1 => y - z = 10;
But, x - 2y + 4z = -1 => -2y + 4z = 0 => -y + 2z = 0;
y - z = 10
-y + 2z = 0
_________(+)__
/ z = 10 => y = 20 .
A. There is one solution. The solution is ( - 1, 20, 10 ).
