The additive inverse of a number is that number with its sign changed.
The additive inverse of 5/8 is -5/8, so that is the value of <em>m</em>.
The sum of a number and its additive inverse is <em>zero</em>. (This is actually the definition of <em>additive inverse</em>.)
5/8 is found at 5/8 on the number line.
m is found at -5/8 on the number line.
"sum" is found at 0 on the number line.