Đáp án:
$\begin{array}{l}
x + y = \frac{{4x - 3}}{5}\\
\Rightarrow y = \frac{{4x - 3}}{5} - x = \frac{{4x - 3 - 5x}}{5} = \frac{{ - x - 3}}{5}\\
Thay\,vào:x + 3y = \frac{{15 - 9y}}{{14}}\\
\Rightarrow x + 3.\left( {\frac{{ - x - 3}}{5}} \right) = \frac{{15 - 9.\left( {\frac{{ - x - 3}}{5}} \right)}}{{14}}\\
\Rightarrow x - \frac{{3x + 9}}{5} = \frac{{15 + \frac{{9x + 27}}{5}}}{{14}}\\
\Rightarrow \frac{{5x - 3x - 9}}{5} = \frac{{75 + 9x + 27}}{{5.14}}\\
\Rightarrow 14.\left( {2x - 9} \right) = 9x + 102\\
\Rightarrow 28x - 126 = 9x + 102\\
\Rightarrow x = 12\\
\Rightarrow y = \frac{{ - x - 3}}{5} = - 3\\
Vậy\,\left( {x;y} \right) = \left( {12; - 3} \right)
\end{array}$
