x= 5 here represents the vertical line passing through the point (5, 0). In fact, every point on the line is of the form (5, y) where y can be any number.
I've posted two problems I'm stuck on. Let's focus on the first. I know that the points of intersection are 0 and 2 and I would think I'd just want to solve this like any other problem of this type but I don't understand what the x=5 represents here?