Chapter 24 - Physical Optics: The Wave of Nature of Light - Learning Path Questions and Exercises - Exercises - Page 840: 1

In the first case, the waves interfere destructively. And in the second case, the waves interfere constructively.

The wavelength $=0.5 m$ So, when the distance is $0.75m$, the path difference is $1.5 lambda$, hence the waves interfere destructively. When the distances differ by $1m$, the path difference is $2lambda$, hence the waves interfere constructively.