Question 1101285
<pre><b>
{{{system(-x - y - z = - 8,-4x + 4y + 5z = 7,2x + 2z = 4)}}}

Solve {{{2x + 2z = 4}}} for x
      {{{2x = 4-2z}}}
      {{{x = 2-z}}}

Substitute 2-z for x in {{{-x - y - z = -8}}}
                        {{{-(2-z) - y - z = -8}}}
                        {{{-2+z-y-z = -8}}} 
                        {{{-2-y = -8}}}  
                        {{{-y = -6}}}
                        {{{y = 6}}}

Substitute 2-z for x and 6 for y in the only equation
we haven't used, which is  {{{-4x + 4y + 5z = 7}}}
                           {{{-4(2-z) + 4(6) + 5z = 7}}}
                           {{{-8+4z + 24 + 5z = 7}}}
                           {{{9z + 16 = 7}}}
                           {{{9z = -9}}}
                           {{{z = -1}}}

Finally find x by substituting -1 for z in {{{x = 2-z}}}
                                           {{{x = 2-(-1)}}} 
                                           {{{x=2+1}}}
                                           {{{x=3}}} 

          (x,y,z) = (3,6,-1)

Edwin</pre></b>