What is 5√28 + √63 in simplest radical form?

The answer is:  "  13√7 " .
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Explanation:
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" √63 "  = √9 √7 = " 3√7 " . 
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" √28  "  = √4 √7 = " 2√7 " .
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"  5√28 " =  " 5 * 2 * √7 = " 10√7 " .
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→   " 5√28 "  +  " √63 "  ;

         =  "  10√7 " + " 3√7 "  ;

         =  "  (10 + 3) *  (√7) " ; 

         =  " 13 √ 7 "
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The answer is:  "  13√7 " .
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