$\begin{equation*} \begin{cases} x = 2y - 3 &\\ 3x + 4y = 1 & \end{cases} \end{equation*}$
3(2y − 3) + 4y = 1
6y − 9 + 4y = 1
10y = 1 + 9
10y = 10
y = 1
x = 2y − 3 = 2 * 1 − 3 = 2 − 3 = −1
Ответ: (−1; 1)

$\begin{equation*} \begin{cases} 8x - y = 5 &\\ -9x + 2y = 4 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} y = 8x - 5 &\\ -9x + 2y = 4 & \end{cases} \end{equation*}$
−9x + 2(8x − 5) = 4
−9x + 16x − 10 = 4
7x = 4 + 10
7x = 14
x = 2
y = 8x − 5 = 8 * 2 − 5 = 16 − 5 = 11
Ответ: (2; 11)

$\begin{equation*} \begin{cases} 2x - 5y = 21 &\\ y = 3x + 1 & \end{cases} \end{equation*}$
2x − 5(3x + 1) = 21
2x − 15x − 5 = 21
−13x = 21 + 5
−13x = 26
x = −2
y = 3x + 1 = 3 * (−2) + 1 = −6 + 1 = −5
Ответ: (−2; −5)

$\begin{equation*} \begin{cases} x - 5y = 4 &\\ 3x - 8y = -2 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} x = 5y + 4 &\\ 3x - 8y = -2 & \end{cases} \end{equation*}$
3(5y + 4) − 8y = −2
15y + 12 − 8y = −2
7y = −2 − 12
7y = −14
y = −2
x = 5y + 4 = 5 * (−2) + 4 = −10 + 4 = −6
Ответ: (−6; −2)