A stone is projected from a horizontal plane. It attains height $H$ & strikes a stationary smooth wall & falls on the ground vertically below the maximum height. Assume the collision to be elastic, find the height of the point on the wall where ball will strike.
tried by assuming that wall is $\frac{3R}{4}$ from the point of throwing and then tried using the equation of trajectory but ans won't come and 
Assuming an elastic collision with the wall this is just a projectile type question.
If instead of hitting the wall the projectile would have continued onward and hit the ground with a trajectory which is the mirror image of that in your diagram.
If the horizontal distance between the point of maximum height and the wall is $d$ then the maximum height was reached after the projectile had travelled a horizontal distance of $2d$ and the wall after travelling a total horizontal distance of $3d$ which I assume is you three quarters of the total range $R$.
Since the horizontal velocity is constant you know that the time taken from maximum height to hit the wall is half the time it takes to go from maximum height to hit the ground.
Having got these times all you now need to do is consider the vertical part of the motion.
