Just draw a rough sketch: two intersecting circles.

Draw the chord -- the line between the two intersections.

Next drawn a line between the two radius points.

It does not need to be accurate to scale. Freehand two circles & two lines.

Then drawn a line from one of the circle intersection points to the center of one of the circles. Call that distance 13 -- the known radius.

You now have a triangle:

1. center of circle to an intersection point.

2. intersection point to 1/2 the chord, or where the chord is intersected by the line connecting the two radii.

3. from the mid point of the chord back to the center of the circle with known radius.

Call the distance between the two circle centers, which is 21, as x, and 21-x.

You now have three sides of a right triangle.

x^2 + (24/2)^2 = 13^2. You can solve for x.

Then use that info to work on the other (unknown) radius.