Draw a circle and draw the chord horizontally across the bottom of the circle. Draw the segment from the center of the circle to the midpoint of the chord. It is 7 m long by the problem's definition and forms a right triangle with the chord. You now have a right triangle and know the lengths of the two legs. The radius of the circle is the hypotenuse of the triangle. Its length can be calculated from the Pythagorean theorem.
c=25
The radius is 25 m.
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