Why is the path difference answer 6.04 cm instead of 3.02 cm?

[apchemstudent]

The answer to the attachment is 6.04 cm. But the intensity should be zero when the difference in path length is half a wavelength.

2sqrt(2)x - 2x = 2.5cm

x = 3.02 cm. However, how come the answer is 6.04cm in stead? Can some one please explain this to me thoroughly? Thanks in advance.

 

[apchemstudent]

can anyone explain this? thanks...

 

[GCT]

The overall phase change due to reflection off the mirrors is 180, thus the path difference needs to be the wavelenth, not 1/2.