Question 1087784
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Nothing can be easier.


<pre>
Multiply the first equation by 2 and then add the result to the second equation.


After multiplying, you will get 

-10x + 10y - 8z =   8,    (1')
 10x -  9y +  z = -20,    (2')
  -x +  9y +  z =   8.    (3')

Now add (1') and (2') and write the sum as an updated second equation. You will get

-10x + 10y - 8z =   8,    (1'')
  0x +   y - 7z = -12,    (2'')
  -x +  9y +  z =   8.    (3'')

Variable x is just eliminated. You can write your system in the form

-10x + 10y - 8z =   8,    (1'')
         y - 7z = -12,    (2'')
 -x  +  9y +  z =   8.    (3'')
</pre>

Done and solved.