Answer:
The correct answer is:  [A]:  " -(-21)  =  21 " .
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Step-by-step explanation:
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Let us examine the answer choices given.
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Choice:  [A]:  " -(-21) = 21 .  This is correct;  
      → since a "negative value of negative value" is a "positive value" ; as shown in this case.  The equal sign holds true, since the
 "negative of a "negative value" is equal to the "positive value" of that same "positive value equivalent."
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Choice: [B]:  " -(-21) = -21 .  This is incorrect.  As explained above:
     → a "negative of negative" is a "positive value"  .
This answer choice shows that a "negative of negative"; is a "negative" :
                                                       → which is:  INCORRECT.
     → Rule out:  "Choice:  [B]."
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Choice: [C]:  " -(21) = 21 .  This is incorrect.  The "negative" value [of a positive integer is not equal to the "equivalent positive value" of that same integer.
    → Rule out:  "Choice:  [C]."
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Choice: [D]:  " (-21) = 21 ";  This is incorrect.  In a conceptually similar way as explained above—(for  "Answer Choice:  [C]" — the "negative value equivalent" of an integer is NOT equal to the "equivalent positive value" [of that same integer.].
   → Rule out:   "Choice:  [D]."
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The correct answer is:  [A]:  " -(-21) = 21 " .
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Hope this is helpful to you! 
    Best wishes!