Question

how does a number line shows that 8 and -8 are opposites

Answer 1

They are both 8 spaces away from 0 in opposite directions.

Answer 2

First of all, opposites are their positive and negative number. For example 4 is the opposite of -4. In total, they have the same amount of spaces from 0. 8 and -8 are 16 spaces from 0.