From the Super Mario Wiki, the Mario encyclopedia
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Mario starts off on the center platform directly in front of the real exit door. He must first ride one of the lower lifts and perform a Handstand Jump onto the center block. He can then ride one of the lifts sliding across the "X" up to the left where there is both a switch and the Key. He can then simply carefully drop down onto the starting platform and unlock the exit door to complete the level.
Pauline's lost items
Within this stage, Pauline's lost items can be found in the following order and in the following places: