Explanation:
x + 2y − z = 3
x + y − 2z = -1
There are three variables but only two equations, so this system of equations is undefined. We cannot solve for the variables, but we can eliminate one of them and reduce this to a single equation.
Double the first equation:
2x + 4y − 2z = 6
Subtract the second equation.
(2x + 4y − 2z) − (x + y − 2z) = (6) − (-1)
2x + 4y − 2z − x − y + 2z = 7
x + 3y = 7