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( 21 x^5 w^3 ) / ( 9 x^2 w^10 )

1) This is just 21/9 which reduces to 7/3

( 7 x^5 w^3 ) / ( 3 x^2 w^10 )

2) Five x's on the top...
    Two x's on the bottom...
algebra problem

Leaves three x's on the top.

( 7 x^3 w^3 ) / ( 3 w^10 )

3) Three w's on the top...  Ten w's on the bottom...  Three will cancel top and bottom leaving seven w's on the bottom.

( 7 x^3 ) / ( 3 w^7 )

Done!