Equation of Family of Circles Which Are Concentric

Q.

Find the equation of the circle concentric with the circle
$x^2+y^2-6x+12y+15=0$
and double of its area.

Q.

Find the equation of the circle concentric with
$x^2+y^2-4x-6y-3=0$
and which touches the y-axis.

Q. The centroid of the triangle formed by 3 co-normal points lies on the axis of parabola.

Q. If $y+3x=0$ is the equation of a chord of the circle, $x^2+y^2–30x=0$, then the equation of the circle with this chord as diameter is

1. $x^2+y^2-3x-9y=0$
2. $x^2+y^2+3x+9y=0$
3. $x^2+y^2-3x+9y=0$
4. $x^2+y^2+3x-9y=0$

Q. The equation of the circle passing through the point (2, 1) and touching y-axis at the origin is

1. $x^2+y^2-5x=0$
2. $2x^2+2y^2-5x=0$
3. $x^2+y^2+5x=0$
4. $x^2-y^2-5x=0$