The general equation of a circle is
where a and b are the x and y components of the center respectively.
In order to get into this format, we have to use a method that I'm drawing a blank on the same, my apologies about that.
Anyway, what you want is to first isolate the variable on side so now you have:
(you'll see why I rearranged this soon)
Now you have to make it into an equation that can be simplified into the format stated earlier.
So, You have to take the x & y terms (-4x & -8y in this case) divide them each by two, and then square them.
The result of this step is the equation:
Notice the new components that have been added (4 & 16) and how the have been added to both sides.
That is an important step.
Now we simplify and get:
So the coordinate of the center is: (2,4) and the radius is 5