Destructive intereference in a wave

[Terp]

Homework Statement

Homework Equations

None really.

The Attempt at a Solution

I know that to make a wave completely destructive to the original I'd have to add pi, so I tried the answer 0+pi*n, which was wrong. I also tried 0 + 3pi*n, which would shift the wave to another destructive position but that wasn't correct either. Would any of you guys know what I'm doing wrong or overlooking? Thanks a lot!

I'm thinking that it may be pi + 2*pi*n, but I want to get some opinions before I try submitting.

 

[Terp]

Bump!