Warp Cannons??

Here's what you do. In world 3 ghost house hit the orange block but do not go to the upper-left door. Instead, go to that switch you hit before and you will see an extra door between the door that stays there and the switch. Go in it. Note: You have to be fast enough and go in the door before it goes away by 1 sec and the music will continue. Go right. The warp cannon is unlocked. World 4: Go on the red blocks in ghost house then go on the pipe. Face the air-breathing ghost. Get your mini 'shroom then go back to the corner of pipe where theres a square, run, jump at the end of the entrance of the pipe and you will find 3 blocks. Jump and you will see a passage and a Star coin. Wall jump and you will get the Star coin. Go inside the door. Go right. You should be able to see a red flag. Go on it. The cannon is unlocked. World 5 ghost house: You should be able to see 3 bricks. Hit the 3 invisible blocks lined up vertically with the bricks. Go on the middle block. If you are Mushroom mario or mario with a powerup that can make you kill enemies duck then jump. A vine comes out. Same with Baby Mario but it is one step less. You will see a boxing ghost. Do not go on the platform until the boxing ghost falls. Go in the door you see. You will be on a yellow elevator. Keep right but try to keep your balance when it comes diagonal. You will fall down really fast then come up really fast. Keep right still. You will find some enemies and the last Star coin. Get the Star coin. Hit a brick and a vine will come out. Climb the vine. Go left. You will see a door. Go in it. Go right with a ghost in pursuit of you. You will see a red flag. Go on it. You have unlocked a cannon. (It is the last one.)

you have to find the secret exit n a level.(most the time it's in a boo house)

Which warp cannon takes you to World 7 in New Super Mario Bros?!?!