Question

A person on top of a 19.6 meter tall building throws a ball straight up and it lands on the ground in 5 seconds. What is the initial velocity?

Answer 1

Using g=10m/s

V=21.08m/s

Answer 2

x = u*t + (1/2)* a* t* t
in this case
x= -19.6
t = 5
a = -9.8

You need to memorise the simple equations describing Newtons Laws of motion (constant acceleration) to be fluent with this stuff.

Others are

v = u + at
v^2 = u^2 + 2ax

Nothing else to it apart from picking the right equation to use.