It sounds like "elementary transformation method" refers to Gauss-Jordan elimination.
Start with the augmented matrix,
• Add (-1)(row 1) to row 2, and (-3)(row 1) to 2(row 3). In other words,
-(2, 3, 1, 1, 0, 0) + (2, 8, 1, 0, 1, 0) = (0, 5, 0, -1, 1, 0)
-3 (2, 3, 1, 1, 0, 0) + 2 (3, 7, 2, 0, 0, 1) = (0, 5, 1, -3, 0, 2)
I'll omit these details in the next several steps. The augmented matrix then transforms to
• Add (-1)(row 2) to row 3:
• Add (-1)(row 3) to row 1:
• Add (-3)(row 2) to 5(row 1):
• Multiply row 1 by 1/10 and row 2 by 1/5:
We're done, and we find that