Question 510115
Start with the basic equation of a circle with center at (h,k) and radius, r.
{{{(x-h)^2+(y-k)^2 = r^2}}} The center is at the origin, so (h,k) = (0,0).  Substitute this.
{{{(x-0)^2+(y-0)^2 = r^2}}} which can be simplified to...
{{{x^2+y^2 = r^2}}} Now to find {{{r^2}}} you use the distance formula: {{{d^2 = (x[2]-x[1])^2+(y[2]-y[1])^2}}} in which you'll find the distance between the circle's center (0,0) and the point contained on the cirle (-2,3).
{{{d^2 = (-2-0)^2+(3-0)^2}}}
{{{d^2 = 4+9}}}
{{{d^2 = 13}}} but this is just {{{r^2}}} so you substitute...
{{{x^2+y^2 = 13}}}