I have reduced a matrix to the following:

$\displaystyle \left[ \begin{array}{rrr|r} -1&1&0&0 \\ 1&-1&0&0 \\ 0&0&0&0 \end{array} \right]$

Obviously $\displaystyle x_1 = x_2$.

How do I figure out what $\displaystyle x_3$ is here?

I did this a few years ago but am lost right now... I have the answer for this problem but I don't understand how to reach it myself.

Cheers to anybody interested!