If the center is in point P(a,b) and has a radius r then circle equation is:
(x-a)²+(y-b)²=r².
In your task we've got P(0,0) so the circle equation is x²+y²=r².
We have to find r. Radius start at (0,0) and end at (-4,-6) therefore
r=√((-4)²+(-6)²) and
r²=(-4)²+(-6)²=16+36=52.
So answer is x²+y²=52.
