Question 744819
Here is what needs to be done:

draw a line which is perpendicular to the cord and intersects the arc, by definition this line bisects the cord

now measure the distance along the perpendicular line between the intersection of the cord and the intersection of the arc, call that distance m ( m must be expressed in mm)

draw two line segments from the cord arc intersection points. where these line segments intersect is the center of the circle  

now you can calculate the radius using the right triangle formula  

(r-m)^2 + (c/2)^2 = r^2 for r (note that c is the length of the cord)