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