slope at a point on a graph

ALright i am stuck on this question, a friend told me to simply draw a line at that point and figure the slope from that, however the answer im getting itsnt working, so heres the question

Using graphical methods, determine at x= 2.8 the value of the derivative of function F(x) plotted in the graph below.

http://img32.imageshack.us/img32/751...09atestnd2.gif

also as a side note, i placed this in the calc forum because the question mentions derivative, if this is the wrong place please let me know.