$\begin{equation*} \begin{cases} x + y = 4 | *(-4)&\\ 4x - 5y = 7 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} -4x - 4y = -16 &\\ 4x - 5y = 7 & \end{cases} \end{equation*}$
−4x − 4y + 4x − 5y = −16 + 7
−9y = −9
y = 1
x + y = 4
x = 4 − y
x = 4 − 1
x = 3
Ответ: (3;1)

$\begin{equation*} \begin{cases} x - y = 6 | *(-2) &\\ 5x - 2y = -3 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} -2x + 2y = -12 &\\ 5x - 2y = -3 & \end{cases} \end{equation*}$
−2x + 2y + 5x − 2y = −12 − 3
3x = −15
x = −15 : 3
x = −5
x − y = 6
y = x − 6
y = −5 − 6
y = −11
Ответ: (−5;−11)

$\begin{equation*} \begin{cases} x - y = -3 | *(-2) &\\ 2x + 7y = 3 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} -2x + 2y = 6 &\\ 2x + 7y = 3 & \end{cases} \end{equation*}$
−2x + 2y + 2x + 7y = 6 + 3
9y = 9
y = 1
x − y = −3
x = −3 + y
x = −3 + 1
x = −2
Ответ: (−2;1)

$\begin{equation*} \begin{cases} 9x + 4y = -2 &\\ x + y = -8 | *(-4) & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} 9x + 4y = -2 &\\ -4x - 4y = 32 & \end{cases} \end{equation*}$
9x + 4y − 4x − 4y = −2 + 32
5x = 30
x = 30 : 5
x = 6
x + y = −8
y = −8 − x
y = −8 − 6
y = −14
Ответ: (6;−14)