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 m.
The sum of a number and its additive inverse is zero. (This is actually the definition of additive inverse.)
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.