Also
\(\left(\begin{array}{r}x1 + x3 + x4\\x2 - x4\\\end{array}\right)=0\)
{x1 = t, x2 = r}
\(\small \left\{ \left(\begin{array}{rrrr}1&0&1&1\\0&1&0&-1\\\end{array}\right) \left(\begin{array}{r}t\\r\\-r - t\\r\\\end{array}\right) = \left(\begin{array}{r}0\\0\\\end{array}\right) \right\} \)
{x3 = t, x2 = r}
\(\small \left\{ \left(\begin{array}{rrrr}1&0&1&1\\0&1&0&-1\\\end{array}\right) \left(\begin{array}{r}-r - t\\r\\t\\r\\\end{array}\right) = \left(\begin{array}{r}0\\0\\\end{array}\right) \right\} \)
{x3 = t, x4 = r}
\(\small \left\{ \left(\begin{array}{rrrr}1&0&1&1\\0&1&0&-1\\\end{array}\right) \left(\begin{array}{r}-r - t\\r\\t\\r\\\end{array}\right)= \left(\begin{array}{r}0\\0\\\end{array}\right) \right\} \)