7 answers
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I assume you meant
√(21x) , and not the way you typed
since 21 = 3x7 , you want either another 3 or 7 in x so that we have at least one perfect square.
none of the first 3 choices contain a 3 or 7 ,but 54 does
so √(21x54)
= √3x7x3x9x2)
= √(81*14)
= 9√14
do the other one the same way