f(x) - this means that a function of x is graphed. So, when there's a number in place of x, like f(2), we need to look at the graph on the x-axis at whatever number is in place of x and go up until we get to where the y-value meets the x.
We are sort of looking for a point - (2, ?)
On this graph, the missing y-value is D. 9.
Hope this helps! :)
