Motion in the vertical direction (take downwards as the +ve direction):

a = 9.8 m/s^2

s = 30 m

u = 0 m/s

t = ?

Find t using

.

Motion in the horizontal direction:

a = 0 m/s^2

u = 15 m/s

t = value calculated above.

s = ?

Find s using

. This is the distance from the base of the cliff,

**which is not the same as the distance of the stone from the thrower.**
If you want the distance of the stone from the thrower, then you need to use Pythagoras' Theorem:

where d is the distance and s is the value calculated above.