Revised SAT Math: Circles in the coordinate plane

 
Try the following SAT math question for the Revised SAT that deals with equation of circles in the coordinate plane and how to manipulate it to obtain the radius of the circle.

The equation of a circle in the $xy$-plane is shown below. What is the radius of the circle?
$$ 36x^2+36y^2-24x-36y=131 $$

1. $\quad \dfrac{1}{2}$
2. $\quad 1$
3. $\quad 2$
4. $\quad 4$