Like I said, most motion graphs have non-straight lines.  Well how do you find the slope of a curve?  You PRETEND that you can straighten that curve out into a flat line.  Take a ruler, or anything straight and hold it next to the curve.  Tilt it around until it seems that if you could flatten the curve, it would be where your ruler is.  Then you get the slope of the line that you add!
The image below shows my attempts at doing this several times.  NOTICE that where the line is already straight, you don't need to make your own line.  The slope of your added line is the speed at that point!!!

Make a photocopy of your first graph assignment.  Add these lines in 4 places, you pick where.  Find the slopes of these lines.