I'm in World 3 of New Super Mario Bros 2 on 3DS. There is a cannon (see image below) which I assume will take me to the second special world. But I have tried every level around it and cannot find any secret exits.

How do I get to the cannon?
1 Answer
As shown in this video (from 1:04)
In the tower as mini mario take the tiny pipe just before the half way point, and follow that branch.