I've been given 3 coordinates: (0,0), (2,0) and (0,2) and I'm asked to draw the circle and then determine the equation of the circle: .

Now, since r=2 and the center point is (0,0), I'd say the equation would be:

, my book tells me it should be (or something like that, don't have it with me). Could someone elaborate on this? I don't get where the -2x,-2y come from!