Aloha :)
$$\begin{array}{rcrcrl}3y&+&9x&=&25,8 &\big|\text{\(x\) und \(y\) vertauschen}\\11x&+&8y&=&-17\end{array}$$$$\begin{array}{rcrcrl}9x&+&3y&=&25,8 &\big|:\,3\\11x&+&8y&=&-17\end{array}$$$$\begin{array}{rcrcrl}3x&+&y&=&8,6 &\big|\cdot\,8\\11x&+&8y&=&-17\end{array}$$$$\begin{array}{rcrcrl}24x&+&8y&=&68,8 &\big|-\text{Gleichung 2}\\11x&+&8y&=&-17\end{array}$$$$\begin{array}{rcrcrl}13x&&&=&85,8 &\big|:\,13\\11x&+&8y&=&-17\end{array}$$$$\begin{array}{rcrcrl}x&&&=&6,6 &\\11x&+&8y&=&-17 & \big|-11\cdot\text{Gleichung 1}\end{array}$$$$\begin{array}{rcrcrl}x&&&=&6,6 &\\&&8y&=&-89,6 & \big|:\,8\end{array}$$$$\begin{array}{rcrcrl}x&&&=&6,6 &\\&&y&=&-11,2&\end{array}$$