It’s easiest if you substitute a variable for 8 so you can see exactly what you’re dealing with; try B.  Basically, you are dividing 3B¹² by 3B⁵:
 3B¹² / 3B⁵
 =B¹² / B⁵
= B^(12-5)
= B⁷
= 8⁷  (D)

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