ANSWER
[tex]11 .6[/tex]
EXPLANATION
x is the radius of the circle.
It is equal to the hypotenuse of the right triangle created by connecting the center and the end of the chord with a straight line.
[tex] {x}^{2} = {5}^{2} + {( \frac{21}{2} )}^{2} [/tex]
[tex]{x}^{2} = 25 + 110.25[/tex]
[tex] {x}^{2} = 135.25[/tex]
[tex]x = \sqrt{135.25} [/tex]
[tex]x = 11.6[/tex]
