Addition and subtraction are commutative
Meaning you can add or subtract in any order because it doesn't change the sum or the difference. So it doesn't change the units.
km/ hr + km/ hr =(km+ km)/hr=2km/hr
i dont think you are wrong for wanting to get even with someone who did you wrong.
(i'm an eye for an eye kind of person) but if this person hurt you in a sexual way, i dont suggest doing anything to him. he should be taken to the proper authorities, you could go to jail depending on what you do. if it was something small like your sibling took your spot on the couch, than yea, i would get even with him by taking one of his favorite snacks lol. but i dont suggest you doing anything illegal.