Initially the sound wave was travelling equal distances from both sides but when the left part was shifted by 5cm then the wave going through the left part has to travel 10cm more. This creates a path difference of 10cm between the two waves.
Now as there has to be minimum sound intensity, thus they interfere destructively and so the path difference has to be a multiple of odd(lambda)/2
Thus the equation we have is :
Putting n=1 we get lambda=20