Question

please solve this question​

Answer 1

So now u have to split your total time of flight in three parts t1, t2, t3
t1 will be the time for reaching the top most height where the final velocity(v) becomes 0, then t2 will be the time to reach the same spot from where the ball is thrown so it will be equal to t1
Then t3 which is the time taken from the height of building to ground where the final velocity will be 3u as mentioned in question
Solving the equation v=u+at and putting the value of u and v and taking care of the direction of g, you will get the final solution as 4u/g, refer my diagram for better understanding