TeX Quellcode:

\begin{array}{rcrcrcrl} 3x_1 &-& 6x_2 &+& 9x_3 &=& 21 \cr -x_1 &+& 2x_2 &+& 4x_3 &=& 21 \cr -x_1 & & &+& 5x_3 &=& 21 \end{array}