Answer
Situation (a): fully constructive interference
Situation (b): fully destructive interference
Work Step by Step
Situation (a):
The phase difference is $~~2~wavelengths$
Fully constructive interference occurs when the phase difference is a multiple of the wavelength.
Situation (b):
The phase difference is $~~1.5~wavelengths$
Fully destructive interference occurs when the phase difference has the form $0.5, 1.5, 2.5,...$ times the wavelength.