The equation of a circle with origin as centre and passing through the vertices of an equilateral triangle whose median is of length $3a$ is

Question

[Hint : Centroid of the triangle coincides with the centre of the circle and the radius of the circle is $\large\frac{2}{3}$ of the length of the median]

$\begin{array}{1 1}x^2+y^2=9a^2\\x^2+y^2=16a^2\\x^2+y^2=4a^2\\x^2+y^2=a^2\end{array} $