Aball is thrown horizontally at a speed of 20 meters per second from the top of a cliff. how long does the ball take to fall 19.6 meters to tht eground

Use one of the equations  of motion under gravity.

s = ut + 1/2 gt^2     where s = distance, u = initial speed , t = time and  g = acceleration due to gravity ( = 9.8 ms^-2)

Initial  downward speed is zero so we have

19.6 = 0t + 1/2 * 9.8 t^2
4.9t^2 = 19.6
t^2 = 19.6/4.9 = 4

t = 2 seconds (answer)