Question 438709
The radius of the smaller circle is 336,while the radius of the bigger circle is 420.  The difference between the two (concentric) radii is 84.  Hence the length of HALF the chord is {{{sqrt(420^2 - 84^2) = sqrt(169344)= 411.514}}}to 3 decimal places.  Multiply by 2 to get the whole length.