Verified answer

Question is unclear so will take a guess at :-

[ 6 x^5 ] [ 21 y ]

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12 x²

21 x³ y

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2

since it is a multiply problem, write it as a single numerator/denominator

(2*3*xxxxx*3*7*y) / (2*2*3*xx)

now cross out common factors of num & denom and write what's left

xxx*3*7*y / 2 which equals 21x³y/2