Taking upwards as positive,
u = 30.0 m/s
s = -15.0m (the ball ends up 15.0m downwards which is negative)
a = -10m/s (this acts downwards which is negative)
t = ?
v = ?
Using the equation s = ut + 0.5at2,
-15.0 = (30.0)t + 0.5(-10)(t2)
-15.0 = (30.0)t + 0.5(-10)(t^2)
5t2+ 30.0t + 15.0 = 0
t2+ 6t + 3 = 0
t = 6.5s (2sf)