The answer would be be B
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You first factor out 54 and thats 3^2 * 6
(The factor of a # is what you multiply to get the answer)
You then do the same to x^9 and y ^7
After that you should have square root of [3^2 * 6x^8 * xy^6 * y]
When you combine this you will get 3x^4 y^3 sqre root [6xy]