Question

if a ball is thrown with speed 29.4 m/s when does it return back to ground?

Answer 1

Strictly speaking there is not enough information in your question.

If the question is
' a ball is projected vertically upwards from ground level at 29.4m/s. Find the time when it returns to the ground'
then you can use the formula fro constant acceleration under gravity

v=u+at

The velocity at its highest point will be 0
The initial velocity is -29.4m/s
Acceleration = g = 9.8 m/s^2

So you can solve how long it takes to reach its highest point.

Since acceleration is constant it will take the same time to fall from that height as it took to get there - so the answer is 2x the time you derived above.