p(-3,-3), q(9,13)

Not sure what the 'p' and 'q' are supposed to mean - is 'p' the center and 'q' a point on the circle?

The general form for a circle is


R^2 = (x-x_0)^2 + (y-y_0)^2


where (x_0, y_0) is the center of the circle and R is the radius. You can get the radius using Pythagorus: if (a,b) is a point of the circle and (x_0,y_0) is the center, then



R^2 = (a-x_0)^2 + (b-y_0)^2


Do you see why that is?