Let's use a simple graphing calculator app.
y = 2x + 6
3y = 6x + 18
We can simplify the second equation to:
y = 2x + 6
We can see that this is the same equation, so what happens?
The two lines lie on top of each other, meaning that we can pick any point that lies on the line. No matter what, it will be a solution. Therefore, this system has infinite solutions.