$\begin{equation*} \begin{cases} 4x - 7y = 30 &\\ 4x - 5y = 90 & \end{cases} \end{equation*}$
4x − 7y − (4x − 5y) = 30 − 90
4x − 7y − 4x + 5y = −60
−2y = −60
y = −60 : (−2)
y = 30
4x − 7y = 30
4x = 30 + 7y
4x = 30 + 7 * 30
4x = 30 + 210
4x = 240
x = 240 : 4
x = 60
Ответ: (60;30)

$\begin{equation*} \begin{cases} -5x + 7y = 6 &\\ 2x + 7y = 76 & \end{cases} \end{equation*}$
−5x + 7y − (2x + 7y) = 6 − 76
−5x + 7y − 2x − 7y = −70
−7x = −70
x = −70 : (−7)
x = 10
2x + 7y = 76
7y = 76 − 2x
7y = 76 − 2 * 10
7y = 76 − 20
7y = 56
y = 56 : 7
y = 8
Ответ: (10;8)

$\begin{equation*} \begin{cases} 3x - 6y = 12 &\\ 3x + 5y = 100 & \end{cases} \end{equation*}$
3x − 6y − (3x + 5y) = 12 − 100
3x − 6y − 3x − 5y = −88
−11y = −88
y = −88 : (−11)
y = 8
3x − 6y = 12
3x = 12 + 6y
3x = 12 + 6 * 8
3x = 12 + 48
3x = 60
x = 60 : 3
x = 20
Ответ: (20;8)

$\begin{equation*} \begin{cases} -3x + 5y = -11 &\\ 8x + 5y = 11 & \end{cases} \end{equation*}$
−3x + 5y − (8x + 5y) = −11 − 11
−3x + 5y − 8x − 5y = −22
−11x = −22
x = −22 : (−11)
x = 2
8x + 5y = 11
5y = 11 − 8x
5y = 11 − 8 * 2
5y = 11 − 16
5y = −5
y = −5 : 5
y = −1
Ответ: (2;−1)