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In this last step, you need to exchange the middle pieces of the last row.
When you have already one side which is complete, make sure that this complete side is facing backwards. Otherwise, just choose the Front face randomly.
Then, decide whether:
- The middle-piece of the front face needs to be exchanged with the middle-piece of the left side (clockwise).
- The middle-piece of the front face needs to be exchanged with the middle-piece of the right side (counter-clockwise).
The result is a completed cube.