Answer
given,
difference between the two consecutive maximum
λ = 0.870 - 0.540
λ = 0.33 m
speed of sound = 340 m/s
b) frequency of the sound
v = f x λ
340 = f x 0.33

f = 1030.3 Hz
a) phase difference
the expression of phase difference is given by




now,



Displacement is the final position of the object minus the initial position of the object.
Xf - Xi. Displacement is not the distance of the object. If you go to the right 10m and to the left another 10m, your displacement is 0m. But your distance is 20m
The statement to every reacting there is, there is a opposite and same reacting.
hope it helps