The distance between two numbers is the absolute value of their difference.
Note that the order doesn't matter, since the distance between A and B is the same as the distance between B and A, obviously:
[tex]d=|-4-5|=|-9|=9[/tex]
[tex]d=|5-(-4)|=|5+4|=|9|=9[/tex]
So, the two points are 9 units away.