The motion of the ball is a composition of two motions:
- on the x (horizontal) axis, it is a uniform motion with initial velocity

- on the y (vertical) axis, it is a uniformly accelerated motion with acceleration
(a) to solve this part, we just analyze the motion on the vertical axis. The law of motion here is

By requiring y(t)=0, we find the time t at which the ball reaches the floor:


(b) for this part, we can analyze only the motion on the horizontal axis. To find how far the ball will land, we must calculate the distance covered on the x-axis, x(t), when the ball reaches the ground (so, after a time t=0.64 s):
B, gossiping is unproessional
Answer
By F = -kx {-ve just indicating the sign of the force}
=>35 = k x (85-50) x 10^-2
=>k = 100 N/m
Again by F = -kx