He is WRONG, WRONG, WRONG, as usual. 

Answer by ikleyn(52866)   (Show Source): You can put this solution on YOUR website!
 . 

The correct solution is THIS:





2x +  y - z = 20    (1)
 x + 3y     = 13    (2)
 x          =  3    (3)


The equation (3) just gives you the solution for x:  x = 3.


Substitute it into equation (2).  You will get

 3 + 3y = 13  ====>  3y = 13 - 3 = 10  ===>  y =  =  .


Thus you found the unknown y, too.
Now substitute x = 3, y =  into equation (1).  You will get

 = 20  ====>  z =  =  -   =  .


Answer.  x = 3,  y =  ,  z =  .




Solved.







Ignore writing by @josgarithmetic, because it leads you in wrong direction.







 

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