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
$\left(x—3\right) \left(x-0\right) + \left(y+ 9\right) \left(y-0\right) = 0$
$x^{2}-3x + y^{2} + 9y=0$
$x^{2} + y^{2}-3x + 9y=0$
