Remember that the circumference of a circle is related to the radius by the relation
C = 2 r. Since you know what the arc length is, the ratio of the arc length to the circumference gives you the angle that the arc subtends (as a portion of the entire angle 360 degree or 2 radians - whichever you prefer).
The chord is the base of an isosceles triangle whose common length sides are the radius of the circle (known) and whose angle between these common sides is also known (from the paragraph above). So if you use the law of cosines, you can calculate the length of the chord.
Hope this helps,
Stephen La Rocque.