Given that $y + 3x = 0$ is the equation of a chord of the circle then $y=-3x$....(i) $\left(x^{2}\right) + \left(-3x\right)^{2}-30x = 0$ $10x^{2}-30x=0$ $10x\left(x-3\right) =0$ $x = 0,\, y=0$ so the equation of the circle is $\left(x?3\right) \left(x-0\right) + \left(y+ 9\right) \left(y-0\right) = 0$ $x^{2}-3x + y^{2} + 9y=0$ $x^{2} + y^{2}-3x + 9y=0$