The question is ambiguous. Does "height" refer to the height of the ball above the top of the cliff or the height of the ball above the sea? Assuming it is "height above the top of the cliff" then using "s= ut+ 0.5at^2" with u= 20 m/s and a= g, the acceleration due to gravity, or "v= u+ at" should give the same thing. Since you are specifically asked for "maximum height" completing the square in the former is more direct. To use the latter, you would have to argue that the ball loses speed until its speed is 0 at the maximum height so solve u+ at= 20 9.8t= 0 to find the time, then use s= u+ 0.5at^2 to find the height
