$\begin{equation*} \begin{cases} 5x - y = 4 &\\ -2x + y = 5 & \end{cases} \end{equation*}$
5x − y − 2x + y = 4 + 5
3x = 9
x = 3
5x − y = 4
y = 5x − 4 = 5 * 3 − 4 = 15 − 4 = 11
Ответ: (3; 11)

$\begin{equation*} \begin{cases} 3x + 5y = 10 &\\ 3x - 7y = 4 |* (-1) & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} 3x + 5y = 10 &\\ -3x + 7y = -4 & \end{cases} \end{equation*}$
3x + 5y − 3x + 7y = 10 − 4
12y = 6
y = 0,5
3x + 5y = 10
3x = 10 − 5y
3x = 10 − 5 * 0,5
3x = 10 − 2,5
3x = 7,5
x = 2,5
Ответ: (2,5; 0,5)

$\begin{equation*} \begin{cases} x + 4y = -7 &\\ x - 9y = 6 |* (-1) & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} x + 4y = -7 &\\ -x + 9y = -6 & \end{cases} \end{equation*}$
x + 4y − x + 9y = −7 − 6
13y = −13
y = −1
x + 4y = −7
x = −7 − 4y = −7 − 4 * (−1) = −7 + 4 = −3
Ответ: (−3; −1)

$\begin{equation*} \begin{cases} 3x - 4y = -5 &\\ 6x + 4y = -1 & \end{cases} \end{equation*}$
3x − 4y + 6x + 4y = −5 − 1
9x = −6
$x = -\frac{6}{9} = -\frac{2}{3}$
3x − 4y = −5
$3 * (-\frac{2}{3}) - 4y = -5$
−2 − 4y = −5
−4y = −5 + 2
−4y = −3
$y = \frac{3}{4}$
Ответ: $(-\frac{2}{3}; \frac{3}{4})$