Part b0 #f'(0)# is the slope of the line tangent to the graph of #f# at #0#. So you need to approximate the slop of the line you drew for part #a#. It looks like the point #(0,-1)# is on the line. Approximate another and finr the slope of the line through those two points.
Part c)
Since #f'(x)# is the slope of the line tangent to the graph at #(x,f(x))#, you need to decide where it looks like the slope of the tangent line is #0#. (Hint: the slope of a horizontal line is #0#.)