Another riddle!
Index, if you're asked to estimate the slope of a line from a graph, then the "estimating" part is simply reading off the coordinates of any two convenient points on the line, as best you can. Once you've got those four numbers, put them into the slope formula.
That's all there is to it.
(Of course, I'm assuming that you know what a tangent line is, and that you can sufficiently draw it on the given graph.)
But, I suspect that you might not have given us all of the information you received.
Did they give you some graph, and you tried to reproduce it for us?
Did they give you some function's definition, and you graphed this function using a machine, and then you tried to reproduce for us what your machine displayed?
If the graph is some such curve through the point (0, 2), then it seems strange, to me, for the posted instruction to make a conditional statement regarding whether or not the slope of the tangent line at that point exists because it obviously exists.
Something doesn't seem right (i.e., a riddle).
Please tell us about everything you received for this exercise.