The opposite of the opposite (or negative) of a number is that number

To write the opposite of a number, a – symbol is used.  For example, the opposite of 5 is -5 (read as "negative 5").  Parentheses are needed to express the opposite of a negative number.  The opposite of –5 is written as –(–5).  Since 5 and –5 are the same distance from 0, the opposite of –5 is 5.  Therefore –(–5) = 5. 

Reference

Mathematics for college students