(- 15,- 12,-9,-6,-3,? 0,? 3,? 6,? 9)

=(0,3,6,9, 12, 15, 18,2 1,24)- 15

=(0, 1,2,3,4,5,6,7,8)*3- 15

=(( 1,2,3,4,5,6,7,8,9)- 1)*3- 15

Remember the order in which (1, 2, 3, 4, 5, 6, 7, 8, 9) are put into the nine squares.

Then subtract 1 from all the numbers to get the order of (0, 1, 2, 3, 4, 5, 6, 7, 8).

Multiply by 3 to get the order of (0, 3, 6, 9, 12, 15, 18, 2 1 24).

Subtract 15 to get the order you want.

All similar problems can basically be deduced by putting (1, 2, 3, 4, 5, 6, 7, 8, 9) in the order of nine squares.

Remember this order.

The result is as shown in the figure:

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