Question 471560
Start at the bottom and go up.
<pre>

               x + y - 3z = 7
                   y - 3z = 4
                        z =-5

1. Start at the bottom with z = -5.

2. Substitute -5 for z in the equation above thst

                   y - 3z = 4
                y - 3(-5) = 4
                   y + 15 = 4
                        y = -11

3. Substitute both -4 for z and -11 for y in the top equation:

              
               x + y - 3z = 7
        x + (-11) - 3(-5) = 7
              x - 11 + 15 = 7
                    x + 4 = 7
                        x = 3

  
(x,y,z) = (3,-11,-5) 
Edwin</pre>