Use the mouse button to indicate where to calculate the derivative. The value of the derivative at that point will be plotted as a point on the graph in green. Mark enough points to identify the shape of the graph of dy/dx as a function of x.
Notice that the derivative curve passes through zero at the point where the function reaches a maximum or minimum. This must be true since it is at these "extrema" (maximum or minimum) where the slope of the tangent to the curve is zero.