A method that always works is to put in points for the x to solve for the y to plot on your graph.

Eg.

If x = -0.5, then y = 4.25

If x = 0, then y = 3

If x = 0.5, theny = 3.75

And then join up the dots

Here's a common image of a cubic graph generated by Wolfram|Alpha:

Your graph should look something like this one, maybe moved up a bit.

Then just add the tangent lines to your graph for the 2 tangent lines.