at x = -1 accurate to within 0.5?
You can change the value of the point of tangency a and the accuracy k desired. View an animation to see how this can be done. In addition, you can make changes to the function y = x3 - 4x. It appears that you must click twice on the graph to get the graphs correctly plotted.
If you get the message "Cannot draw this graph ..." in the notebook, just move the cursor onto the graphing area and click the left mouse button.