Answer
a. 240 m.
b. 120 m.
Work Step by Step
The path difference is 120m.
a. The sound will be canceled when the path difference is an odd multiple of half wavelengths.
Destructive interference occurs for $120m=\lambda/2, 3\lambda/2, 5\lambda/2, 7\lambda/2, …$
The longest wavelength is when $120m=\lambda/2$, or $240m=\lambda$.
b. The sound will be maximally reinforced when the path difference is an integer multiple of wavelengths.
Constructive interference occurs for $120m=0, \lambda, 2\lambda, 3\lambda, 4\lambda, …$
The longest wavelength is when $120m=\lambda $.
