$\begin{equation*} \begin{cases} y = 9x + 5, &\\ y = -6x - 25; & \end{cases} \end{equation*}$
9x + 5 = −6x − 25
9x + 6x = −25 − 5
15x = −30
x = −30 : 15
x = −2
y = 9x + 5
y = 9 * (−2) + 5 = −18 + 5 = −13
Ответ: (−2;−13)

$\begin{equation*} \begin{cases} y = 13x - 7, &\\ y = 23x - 6; & \end{cases} \end{equation*}$
13x − 7 = 23x − 6
13x − 23x = −6 + 7
−10x = 1
x = 1 : (−10)
x = −0,1
y = 13x − 7
y = 13 * (−0,1) − 7 = −1,3 − 7 = −8,3
Ответ: (−0,1;−8,3)

$\begin{equation*} \begin{cases} y = -8x - 15, &\\ y = 5x + 24; & \end{cases} \end{equation*}$
−8x − 15 = 5x + 24
−8x − 5x = 24 + 15
−13x = 39
x = 39 : (−13)
x = −3
y = 5x + 24
y = 5 * (−3) + 24 = −15 + 24 = 9
Ответ: (−3;9)

$\begin{equation*} \begin{cases} y = -11x + 9, &\\ y = -21x + 11. & \end{cases} \end{equation*}$
−11x + 9 = −21x + 11
−11x + 21x = 11 − 9
10x = 2
x = 2 : 10
x = 0,2
y = −11x + 9
y = −11 * 0,2 + 9 = −2,2 + 9 = 6,8
Ответ: (0,2;6,8)