Rather than give you the answer, here is a big hint that should help you out.
Part of the 7-4 platforms are split in to 3 levels (upper, middle and lower), you need to be on the right platform level in the right order. If you do it correctly you will here a 'ding' sound - the same as when collecting a coin. If you are on the wrong platform level, you will not here the sound and the level will repeat. Simply use trial and error and listen for the 'ding' and you'll get through.
Hope that helps.
01/01/2008
Super Mario Galaxy
Melbourne, Victoria