How do you find the derivative of a graph?

Without being given the function of f(x) and g(x) (just have the graph) how am I supposed to find the derivative at a particular point.
This is the website for the problem it is at the very bottom #82. Thank you
http://learn.flvs.net/educator/temp/lullmann1/2161/Student_Text_Files-8th_Edition/Module_2_Text_Files/332460_0203.pdf