The blue line in the graph indicates the function f(x).
The values in the y-axis are the value of the function, that is, the value of f(x) for a given value of x. The x-axis indicates what value of x generates the value in the y-axis.
So, if we want to find the value of x that gives us f(x) = 7, we need to find where is the value '7' in the y-axis, then we draw an horizontal line from this value toward the line of the function (blue line).
This horizontal line will intersect the function in a certain point. This point is where the function has the value 7.
Now, to find the value of x of this point, we draw a vertical line from this point downwards, until it intersects the x-axis.
This way, looking at the image, we can see that the value of x that gives us f(x) = 7 is the value x = 5.