Giúp mình bài này với các bạn

Đáp án: $\begin{array}{l}
a)\left( {x;y} \right) = \left( {10;1} \right)\\
b)\left( {x;y} \right) = \left( {\dfrac{{11}}{5};\dfrac{7}{5}} \right)\\
c)\left( {x;y} \right) = \left( {1;1} \right)
\end{array}$

Giải thích các bước giải:

$\begin{array}{l}
a)\left\{ \begin{array}{l}
x - 3y = 7\\
2x - 5y = 15
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
2x - 6y = 14\\
2x - 5y = 15
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
2x - 5y - 2x + 6y = 15 - 14\\
x - 3y = 7
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
y = 1\\
x = 3y + 7 = 10
\end{array} \right.\\
Vậy\,\left( {x;y} \right) = \left( {10;1} \right)\\
b)\left\{ \begin{array}{l}
3x - 4y = 1\\
2x - y = 3
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
3x - 4y = 1\\
8x - 4y = 12
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
5x = 11\\
2x - y = 3
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
x = \dfrac{{11}}{5}\\
y = 2x - 3 = \dfrac{7}{5}
\end{array} \right.\\
Vậy\,\left( {x;y} \right) = \left( {\dfrac{{11}}{5};\dfrac{7}{5}} \right)\\
c)\left\{ \begin{array}{l}
3x + 5y = 8\\
7x - 2y = 5
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
6x + 10y = 16\\
35x - 10y = 25
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
41x = 41\\
7x - 2y = 5
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
x = 1\\
2y = 7x - 5 = 2
\end{array} \right.\\
 \Leftrightarrow \left\{ \begin{array}{l}
x = 1\\
y = 1
\end{array} \right.\\
Vậy\left( {x;y} \right) = \left( {1;1} \right)
\end{array}$