This is a one dimensional kinematics problem. We know the time, acceleration, and distance covered. We want to know the initial velocity. Here we would use the first kinematic equation.
Since the rock hits the river we can consider the final position y=0, y0=30 a=-9.8 m/s2 t=2.0s
simplifying this we can rewrite this as
0=y0+v0*t-(1/2)gt2 or v0*t=(gt2)/2-y0
Getting a common denominator we get
v0t=(gt2-2y0)/2 or v0=(gt2-2y0)/2t
Pluging in the numbers we get
or 5.2 m/s downward.