Question 902277
1,1,9
4,3,30


add  down (-4) *row 1 to row 2
1,1,9
0,-1,-6

divide row 2 by -1
1,1,9
0,1,6

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

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

final
1,0,3
0,1,6

1,0,3
0,1,6

"3","6"
(3,6)
*[invoke solving_linear_system_by_elimination 1,1,9,4,3,30]