Minsui, I'm sorry you haven't gotten a response.  It is hard to answer your question, because we cannot see the function the question is asking about and we never see where the 7 appears in the function.

Keep in mind that, although we usually think of x as horizontal position and y as vertical position or height, this is not always correct.  There are many things we graph that might come out differently.  The horizontal axis represents the independent variable--what we observe or measure.  The vertical axis is a dependent variable--what we calculate or what we find corresponds to the value of the horizontal axes.

There are many situations where a vertical position would be an independent variable and would be plotted on a horizontal axis; for example, if we measure air pressure versus height.  We would plot it as (h, p).

In the formula they give, 

y=a(x-h)^2 + k,

k will represent the vertical position of the maximum point in distance above the ground.  h will represent the _positive_ displacement of x, or how far away from x horizontally the maximum point will occur.