+129899 Note, NSS, that drawing a radius to either end of the chord will produce a right triangle with legs of 5 and (1/2)21 = 10.5 [the fact that the segment marked a "5" meets the chord at a right angle means that the chord is bisected at that point of intersection....and the bisected part forms the other leg]
So the hypotenuse of this triangle will equal the radii, x
x = sqrt ( 10.5^2 + 5^2 ) = sqrt (135.25) ≈ 11.6
[ Looks like you got the correct answer....!! ]
