Question 638214
second system
b = 7 
now get a which is -3

6,5,17
-6,1,25

divide row 1 by 6/1
1,5/6,17/6
-6,1,25

add  down (6/1) *row 1 to row 2
1,5/6,17/6
0,6,42

divide row 2 by 6/1
1,5/6,17/6
0,1,7

We now have the value for the last variable.
We will work our way up and get the other solutions.

add up  (-5/6) *row 2 to row 1
1,0,-3
0,1,7

final
1,0,-3
0,1,7

1,0,-3
0,1,7