We are given that a and b are the same distance from zero.
By representing a and b as directed distances on the number line.
If we start at zero and move a units to the right, and then move the same number of units to the left, we will be back at 0.
Since a and b are the same distance from zero but are on opposite of zero,so we can write b = -a.
Sum of the number in opposite side is always zero.
[tex]\begin{gathered} a+b=a+(-a) \\ a+b=a-a \\ a+b=0 \end{gathered}[/tex]
