Go to Bowser junior's tower at the 2nd star coin at the door DO NOT go into the door. Jump over the two moving blocks on the right and smash through the blocks with a shell to the red pipe. Enter the pipe to the red flag.
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look up super Mario bros Cannon world 1 and level 1
in world 1 at the tower there is a red flag that you need to get to unlock the cannon.
you cant
you can unlock a level with a pipe and cannon around it (i got it on a red flag on world 2 number 4 ) and finish that on a cheat flag( the yellow pipe above the other 1). this will unlock the cannon.
There are hidden exits on some levels, and when completed, will open up the warp cannon. Warp cannons will transport you 4 worlds ahead, for example, the warp cannon in world 1 will transport you to world 5.