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)
