#### Answer

The solution set is (0,0,0).

#### Work Step by Step

Starting with the original matrix
1 -4 9 0
0 1 7 0
0 0 2 0
We do the following row operations:
divide the last row by 2
1 -4 9 0
0 1 7 0
0 0 1 0
subtract 7 times row 3 from row 2
1 -4 0 0
0 1 0 0
0 0 1 0
add 4 times row 2 to row 1
1 0 0 0
0 1 0 0
0 0 1 0
And the solution set is the last entries in each row , in order.