Initial velocity of ball is u=19.6ms−1, final velocity v=0.
Using v=u+at, here, acceleration a=−g.
Time taken in upward motion above tower is ta=gu=9.819.6=2s
Now, time taken from top most point to ground is td=6−2=4s.
Or, td=g2hmax (Using s=ut+21gt2)
⇒hmax=216×9.8=5392m.
Hence, the value of k=392.