$\begin{equation*} \begin{cases} y = 10x + 30 &\\ y = -12x + 272 & \end{cases} \end{equation*}$
10x + 30 = −12x + 272
10x + 12x = 272 − 30
22x = 242
x = 242 : 22
x = 11
y = 10x + 30 = 10 * 11 + 30 = 110 + 30 = 140
Ответ: (11;140)

$\begin{equation*} \begin{cases} y = -18x + 25 &\\ y = 15x + 14 & \end{cases} \end{equation*}$
−18x + 25 = 15x + 14
−18x − 15x = 14 − 25
−33x = −11
$x = \frac{11}{33} = \frac{1}{3}$
$y = 15 * \frac{1}{3} + 14 = 5 + 14 = 19$
Ответ: $(\frac{1}{3};19)$

$\begin{equation*} \begin{cases} y = 15x - 21 &\\ y = 7x - 77 & \end{cases} \end{equation*}$
15x − 21 = 7x − 77
15x − 7x = 21 − 77
8x = −56
x = −56 : 8
x = −7
y = 7x − 77 = 7 * (−7) − 77 = −49 − 77 = −126
Ответ: (−7;−126)

$\begin{equation*} \begin{cases} y = -7x - 19 &\\ y = 14x - 1 & \end{cases} \end{equation*}$
−7x − 19 = 14x − 1
−7x − 14x = −1 + 19
−21x = 18
$x = -\frac{18}{21} = -\frac{6}{7}$
$y = -7x - 19 = -7 * (-\frac{6}{7}) - 19 = 6 - 19 = -13$
Ответ: $(-\frac{6}{7};-13)$