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This is really easy... and it's going to be really important in the subtractions lessons for signed numbers.

The opposite of a number is just the number on the opposite side of zero on the number line.

The opposite of a is -a.
The opposite of
-a is a.

Check it out:

number line with -5 and 5 highlighted

The opposite of 5 is -5.

The opposite of -5 is 5.

 

number line with -3 and 3 highlighted

The opposite of -3 is 3.

The opposite of 3 is -3.