\documentclass[11pt,dvips]{article}
\begin{document}

\begin{LARGE}

$$
\begin{array}{rcr}
-i_1  +  i_2              +  i_6  & = &  i_S \\
i_1          +  i_3           -  i_5          & = &  0 \\
      -  i_2  +  i_3  +  i_4                   & = &  0 \\
R_1 i_1  +  R_5 i_2  +  (R_2 + R_3) i_3             & = &  v_S1 \\
      -  (R_2 + R_3) i_3  +  R_6 i_4  +  R_4 i_5      & = &  v_S2 \\
    R_5 i_2      -  R_6 i_4      +  R_7 i_6  & = &  0 \\
\end{array}
$$
\end{LARGE}

\end{document}
