Yeah, what Sol Apollo said. If you want an easy way to find the person to rep them back you can click on their user name, go to their profile and search for all the posts they've made. This way you can find a post of theirs and click on the box on their post with the person's head in it that lets you rep them back
Repping back is always nice.
If you don't want to rep back, you don't really have to do anything and no one will hate you for it