==> Their sum is a wave with amplitude greater than either of
the original waves . . . Constructive interference
==> Their sum is a wave with amplitude less than either of
the original waves . . . Destructive interference
We will define the Total mass to calculate the force, so our values are:
Total Mass 
The Weight is,

Through the hook's Law we calculate X.
, where x is the lenght of compression and K the Spring constant.
We don't have a K-Spring, but we can assume a random value (or simply let the equation in function of K)

I assume a value of 

Answer:
5.5 m/s
Explanation:
Assuming the bottle is initially stationary, we can write the law of conservation of momentum as follows:

where
is the mass of the ball
is the initial velocity of the ball
is the mass of the bottle
is the final velocity of the bottle
is the final velocity of the ball
Solving for
,

Answer:
I think it is 5.6. This is my answer
Here's a perfect example of a stable and unstable system.