\left[ {\begin{array}{*{20}c}
 {\sum\limits_{i = 1}^m {x_i^2 } } & {\sum\limits_{i = 1}^m {x_i } } \\
 {\sum\limits_{i = 1}^m {x_i } } & {\sum\limits_{i = 1}^m 1 } \\
\end{array}} \right]\left[ {\begin{array}{*{20}c}
 a \\
 b \\
\end{array}} \right] = \left[ {\begin{array}{*{20}c}
 {\sum\limits_{i = 1}^m {x_i y_i } } \\
 {\sum\limits_{i = 1}^m {y_i } } \\
\end{array}} \right]