Question 1091559
. 
The correct solution is THIS:


<pre>
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 = {{{10/3}}} = {{{3}}} {{{1/3}}}.


Thus you found the unknown y, too.
Now substitute x = 3, y = {{{10/3}}} into equation (1).  You will get

{{{20/3 + 3 -z}}} = 20  ====>  z = {{{20 - 3 - 20/3}}} = {{{17}}} - {{{6}}} {{{2/3}}} = {{{10}}} {{{1/3}}}.


<U>Answer</U>.  x = 3,  y = {{{3}}} {{{1/3}}},  z = {{{10}}} {{{1/3}}}.
</pre>

Solved.



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