the string experiences max tension at the lowest point,
By Work energy theorem, the min speed required at the lowest point to complete a vertical circle is
where L is the length of the string, corresponding to this the max tension comes out to be 6mg=15N, thus we can say that the ball will never be able to complete the vertical circle,
this is the actual scenario, and the question is irrelavent.
but if need be, to get an answer,
at the top point,
T + mg = m
/r
14 + 15 = 0.25*
*2
= v