First, get one of the variables on its own. I'm going to do the first equation since it looks easier. :)
![3x + 6y = 18](https://img.qammunity.org/2019/formulas/mathematics/college/xutdxyzeybuofzyq05uzfsqzc8ns9hhcnq.png)
Subtract 6y from both sides.
![3x = 18 - 6y](https://img.qammunity.org/2019/formulas/mathematics/college/ohtn8hgenjg5ts4bxo2fdyeiu89txihvsc.png)
Divide both sides by 3 to get the variable alone.
![x = 6 - 2y](https://img.qammunity.org/2019/formulas/mathematics/college/arahyryn54g1850djm69a9l5zesxwybeck.png)
Plug this into the other equation for x.
![3y = - (3)/(2) (6 - 2y) + 9](https://img.qammunity.org/2019/formulas/mathematics/college/lgqjd0gmcr7z61gcvab3a1uen28s9jk9ys.png)
Distribute.
![3y = - 9 + 3y + 9](https://img.qammunity.org/2019/formulas/mathematics/college/jinlmrr5ijm2xist3bjfad55ia493vk4d4.png)
Add like terms.
![3y = 3y](https://img.qammunity.org/2019/formulas/mathematics/college/rzv22zfjdfkvowg29us8p8rl0gle60jdxn.png)
The system has infinitely many solutions because both sides equal each other.
Hope this helped!
:)