Find the equation of a circle that intersects the $y$-axis at the origin and at the point $(0,6)$ and also touches the $x$ axis.
Okay, so I wasn't sure how to do this so I looked at the answer at the back of the book and decided to graph it. The book says the answer is $x^2+y^2-6y=0$ and when graphed it shows that the line joining $(0, 0)$ and $(0, 6)$ passes through the center of the circle and is a diameter. Knowing this made solving the question very easy.
So what I'm kinda confused about is how you would have known, just from the information given in the question or by other methods, that the line joining $(0, 0)$ and $(0, 6)$ was a diameter.
Any other solutions/hints would be much appreciated.