I think I know what you are thinking and you are halfway on the right track. : ) You are probably looking at the x-axis right in between the pentagon and its reflection. This is the right line, but it is a y-line and not x. When we say a line is at x = 0, we mean that every point on that line has an x-value of 0, such as (0,1), (0,2), (0, 3) etc. Any line in the form x = 0 (or any other number) is a vertical line. Y-lines are the same way but they run horizontal like the line of reflection in this problem. : )
