Question 200112
First moving the constant term to the right side, we have
{{{x^2+y^2+2x-4y=4}}}
Re-arranging the terms on the left side, we get
{{{x^2+2x+y^2-4y=4}}}
Add 1 and 4 to both sides, we have
{{{x^2+2x+1+y^2-4y+4=4+1+4}}}
{{{(x+1)^2+(y-2)^2=9}}}
{{{(x+1)^2+(y-2)^2=3^2}}}

So the center of the circle is (-1, 2). The radius is 3.