$\begin{equation*} \begin{cases} x + y = 8 &\\ 3x - 2y = 9 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} x = 8 - y &\\ 3(8 - y) - 2y = 9 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} x = 8 - y &\\ 24 - 3y - 2y = 9 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} x = 8 - y &\\ -5y = 9 - 24 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} x = 8 - y &\\ -5y = -15 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} x = 8 - 3 &\\ y = 3 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} x = 5 &\\ y = 3 & \end{cases} \end{equation*}$
Ответ: (5; 3)

$\begin{equation*} \begin{cases} 2x + 5y = 13 &\\ 3x - 5y = -13 & \end{cases} \end{equation*}$
2x + 5y + 3x − 5y = 13 − 13
5x = 0
x = 0
2 * 0 + 5y = 13
5y= 13
$y = \frac{13}{5} = 2\frac{3}{5}$
$\begin{equation*} \begin{cases} x = 0 &\\ y = 2\frac{3}{5} = 2,6 & \end{cases} \end{equation*}$
Ответ: (0; 2,6)