Step-by-step explanation:
Constructive interference takes place when the path difference is a whole number of wavelengths, example, 1λ , 2λ , 3λ. Similarly the phase difference will be 2π, 4π, 6π radians.
Destructive interference takes place when path difference is an odd number of half wavelengths, example, λ/2 , 3λ/2, 5λ/2. Similarly the phase difference will be π, 3π, 5π