$\begin{equation*} \begin{cases} 3x - 2y = 12 &\\ x + 2y = -4 & \end{cases} \end{equation*}$
3x − 2y + x + 2y = 12 − 4
4x = 8
x = 2
x + 2y = −4
2 + 2y = −4
2y = −4 − 2
2y = −6
y = −3
Ответ: (2; −3)

$\begin{equation*} \begin{cases} 3x - y = 4 &\\ 2x + 3y = 21 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} y = 3x - 4 &\\ 2x + 3y = 21 & \end{cases} \end{equation*}$
2x + 3(3x − 4) = 21
2x + 9x − 12 = 21
11x = 21 + 12
11x = 33
x = 3
y = 3x − 4
y = 3 * 3 − 4
y = 9 − 4
y = 5
Ответ: (3; 5)

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

$\begin{equation*} \begin{cases} 4x + 3y = 10 &\\ x - 2y = -3 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} 4x + 3y = 10 &\\ x = 2y - 3 & \end{cases} \end{equation*}$
4(2y − 3) + 3y = 10
8y − 12 + 3y = 10
11y = 10 + 12
11y = 22
y = 2
x = 2y − 3
x = 2 * 2 − 3
x = 4 − 3
x = 1
Ответ: (1; 2)