Circles

15.

Since the  diameter is perpendicular to the chord AB,  then it bisescts  AB....thus.....AE  = BE  =  6

And the intersecting  chord theorem says this

DE * CE  = AE  * BE

DE  *  2  =   6 * 6

DE  * 2   = 36     divide both sides by 2

DE  = 18

So....the diameter  =  DE + CE  = 18  + 2   =  20

And the radius is  1/2 of this  = 10

19....

Since  minor arc MN  = minor arc  QP......then the chords span  equal arcs......and, by Euclid, chords spanning equal arcs  are equidistant from the center of a circle