A stone is thrown upwards with speed v from the top of a tower. . .

Question : A stone is thrown upwards with speed v from the top of a tower. It reaches the ground with a velocity 3v. What is the height of tower?

Doubt by Zoha

Solution : 

Let the height of the building (s) = h

Here 

Initial Velocity (u) = v
Final Velocity (v) = 3v

Using III equation of motion

v²-u²=2as
(3v)²-(v)²=2(-g)h

9v²-v²=-2gh
8v²=-2gh
-8v²/2g=h
-4v²/g=h

h=-4v²/g
Here -ve sign shown that the displacement is in the opposite direction as compared to direction in which the ball is thrown. 

Height of the tower is 4v²/g.