That's correct!

b) This is easy. Just verify that (0, 0) lies on y = -2x.

c) The "circle passing through AB?" Do you mean that AB is the circle's diameter? If so then you know the center of the circle, it's your midpoint of AB. Then to find the radius find the length of the line segment between the midpoint and point A (or point B, it will be the same.) Then plug that into the equation for a circle.

-Dan