The capacitance of the capacitor is 
Explanation:
The capacitance of a parallel-plate capacitor is given by the equation

where
k is the dielectric constant of the medium
is the vacuum permittivity
A is the area of the plates
d is the separation between the plates
For the capacitor in this problem, we have:
k = 2.1 is the dielectric constant
is the separation between the plates
(I assumed that 5.4 m is a typo, since it is not a realistic size for the side of the plate)
Therefore, the capacitance of the capacitor is

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Complete Question
A commuter train passes a passenger platform at a constant speed of 39.6 m/s. The train horn is sounded at its characteristic frequency of 350 Hz.
(a)
What overall change in frequency is detected by a person on the platform as the train moves from approaching to receding
(b) What wavelength is detected by a person on the platform as the train approaches?
Answer:
a

b

Explanation:
From the question we are told that
The speed of the train is 
The frequency of the train horn is 
Generally the speed of sound has a constant values of 
Now according to dopplers equation when the train(source) approaches a person on the platform(observe) then the frequency on the sound observed by the observer can be mathematically represented as

substituting values


Now according to dopplers equation when the train(source) moves away from the person on the platform(observe) then the frequency on the sound observed by the observer can be mathematically represented as

substituting values


The overall change in frequency is detected by a person on the platform as the train moves from approaching to receding is mathematically evaluated as



Generally the wavelength detected by the person as the train approaches is mathematically represented as



<h2>
Answer:</h2>
<em>Hello, </em>
<h3><u>QUESTION)</u></h3>
<em>✔ We have : λ = T x v ⇔ v = λ x f</em>
- v = 500 x 0,68
- v = 340 m/s
The speed of this sound wave is therefore 340 m/s.