It would be: (y2-y1) / (x2-x1) = (9-(-9)/5-5 = 18/0 = undefined
OPTION D IS YOUR ANSWER.........
To find the answer I just added all of the dots after the three (amount of books read) and it is equal to 7. :D hope this helped :D
See the diagram below.

is the height, and

is the slant height.
The height is the distance from the
base of the pyramid to the vertex (the top of the pyramid). It runs perpendicular to the base.
The slant height is the distance from the
base of the triangles which make up its sides to the vertex. It runs perpendicular to this base as well.
<h3>
<u>Explanation</u></h3>
f(a) means the value of f(x) is ... when x = a. That means if we substitute x = 3, we would get f(3).
f(3) also means the value of f(x) is ... when x = 3.
f(x) can also be defined as y // f(x) = y
You can find the value of f(x) at specific domain from the graph by looking at x = 3 then look up to where the point or where the graph passes. From the graph, when x = 3 as we look up and the graph passes y-coordinate at 1.
Therefore we can say that when x = 3, y = 1.
<h3>
<u>Answer</u></h3>
f(3) = 1