The equation of a circle is:
(x-h)2 + (y-k)2 = r2
where (h,k) is the location of the center and r is the radius. If the circle is cnetered at the origin, (0,0), then its equation is:
(x-h)2 + (y-k)2 = r2
(x-0)2 + (y-0)2 = r2
x2 - y2 = r2
Now we just need to find the value of r, the radius. The point (-3,0) is on the circle so the distance from the center (0,0) to (-3,0) is equal to the radius. Can you finish it?