The equation for any circle centered at the origin with a radius r is the following:

By looking at the picture, we can see that the radius is 5. We can also use the distance formula to find the distance between (0,0) and (-3,-4), and we will also get 5.
Thus, r=5.
This makes the equation:


That should be your answer, let me know if this is confusing in any way.
Happy Studying~