$b)$
Theo hệ thức Vi - et:
$\left\{ \begin{array}{l} {x_1} + {x_2} = 2m\\ {x_1}{x_2} = m - 7 \end{array} \right.\\$
$<=> \left\{ \begin{array}{l} {x_1} + {x_2} = 2m\\ 2{x_1}{x_2} = 2m - 14 \end{array} \right.\\$
$<=> {x_1} + {x_2} - 2{x_1}{x_2} = 2m - 2m + 14\\$
$<=> {x_1} + {x_2} - 2{x_1}{x_2} = 14\\$
$<=> {x_1}\left( {1 - 2{x_2}} \right) + {x_2} = 14\\$
$<=>$`x_1=(14-x_2) /(1-2x_1)`
