D. 5.098 x 106 is the correct answer when reduced to the proper notation.
Answer:
Part a)

Part b)

Explanation:
As we know that the observer is standing in front of one speaker
So here the path difference of the two sound waves reaching to the observer is given as


now phase difference is related with path difference as


here in order to find the wavelength


now we have

Part b)
Now we know that when phase difference is odd multiple of 
then in that case the the sound must be minimum
So nearest value for minimum intensity would be

so we have

so we have

now we have


Mechanical weathering is basically the process of breaking big rocks into little rocks.