Here is the question: The figure below shows the graph of f', the derivative of the function f, on the closed interval from x=-2 to x=6. The graph of the derivative has horizontal tangent lines at x = 2 and x = 4. Find the x-value where f attains its absolute maximum value on the closed interval from x = -2 to x = 6. Justify your answer.

Can someone explain how I would go about trying to find the maximum value? Is there a certain way to go about doing it when given a graph rather than a function? I understand how to go about it when given just a plain function; however, I can't seem to figure how to do it when just given the graph. Can you explain how I would begin to solve a question like this one?

Here is the graph:

\f